Plants seemed to have thrived for millions of years without any fertilizer being added. Why can't I just not use fertilizer?
You certainly can try to grow a plant without using additional fertilizers. I think it's a fun experiment to grow the same plant using multiple setups to see what effect is had on the plant.
To answer your question, yes the plant will need certain nutrient to thrive, but you don't necessarily have to give it Miracle Grow. On most plant food containers you'll find three numbers. It'll look something like '10-10-10', '20-10-20', '12-8-10', '5-0-0', etc... These three numbers represent Nitrogen, Phosphorus (sometimes called phosphate), and Potassium (K being the periodic table representation for it, or potash). These are three of the big elements needed to help the plant grow. There are also a host of micro nutrients needed as well.
There are lots of explanations that can give better detail than I can on the effects of these three elements, but basically they affect plant growth, root growth, and flower/fruit growth. So if you see a number like '5-0-0', then it is only trying to affect one of these.
So, in the wild, there is balanced ecosystem where plants and animals die and feed there nutrients into the soil. Leaves drop and wood rots, insects and worms break it down and transform it and it creates a cycle. Or you have the opposite effect where water washes almost all nutrients away and plants such as 'carnivorous plants' have to trap insects and 'digest' them to obtain these nutrients. Also, Darwinism comes into play from the woods to your back yard. If you find a cute little flower growing at the edge of your yard and you dig it up and pot it for your enjoyment, you could very well kill it. In the spot it was in, it very well may be shaded most of the day, or maybe there are ornamental stones that divert more or less water to it. Basically, seeds get scattered everywhere and they only survive where the conditions are optimal. So when people take them from the habitat they're adapted to, then they have to provide those same conditions to the plant.
Each plant is different, but a lot of the plants you can pick up at your local plant store are either hardy or common to your area, so you have a buffer in taking care of them. Many potted house plants get fertilized with chemical fertilizers, such as 'miracle grow', because when a plant is watered, chemicals are leached out. They are also taken out of the soil by the plant. The miracle grow, uses the water to put them back into the soil for the plant to use, the same as nature is putting them back into the soil for your little yard plant or the trees in the forest.
I called it a 'chemical fertilizer', but in reality every spec of fertilizer is a chemical, it's just about how they are made. When you see something that says 'organic' vs 'non-organic' it just means that on the 'organic' no human made chemicals were applied, but those same chemicals can usually be applied in a weaker concentration by natural means. For instance, people compost, vermicomposter for worm castings, add wood ash, etc... The Indians used to put a dead fish under their plants to help fertilize them. These are all eco-friendly ways to fertilize your plant. I encourage you to check them out. I'm currently setting up my first vermicomposter.
So, yes, I do think it's necessary to use fertilizer. However, I do encourage you to experiment. Take three of the same pots and plants. Use whatever dirt you want for one, miracle grow potting soil (which contains a slow release chemical fertilizer by the way) for another, and compost, which you can buy at any garden center (possible mixing in 20-30% worm castings). Then you begin your experiment and see which does better. Ideally, you should water them evenly, but that can be another experiment, because if you water them all equally then your moisture control soil will either be too wet, or you'll dehydrate and kill the other two. Water as needed and document your results.
You'll probably notice that the regular dirt and plain water doesn't grow much and gets weak. The miracle grow plant will probably grow much faster and may even get tall and leggy. The compost will grow slower, but will probably be healthier and possible tastier if it's a fruit/veggie. It really depends on what effect you're going for, but record your results and review them at the end of the summer, or whatever time period you set, though I wouldn't go less than 3 months. It's important to learn what works and what doesn't. A person who is told how to do something will be less knowledgeable than a person who experiments and sees a direct correlation between an action and a consequence. Like, not fertilizing leads to a weak and dying plant.
Over fertilizing can be bad as well. You'll see many people, even on this site, ask why their plant is growing so well, but not producing any fruit. They've been fertilizing the crap out of it and they don't understand. What's happened is that they plant is in growth mode and it's putting all it's energy into that growth. If they cut back on the fertilizer, then they typically see the fruit they want. So if your miracle grow soil says it's good for 3 months, then don't use additional fertilizer till after that, and then only the recommended amount. Good luck and I hope this answers your question.
Joel Salatin become famous in organic farming circles for saying he isn't a cattle farmer, he's a grass farmer. The idea is that if he grows healthy fodder, his cattle will be healthy.
There is a similar idea for gardening that if you are building healthy soil and using good management practices for the land you garden, your plants will do their thing on their own. I think it has a lot of merit. Many who take this approach think of many added fertilizers as counterproductive, as they are not helpful to the soil biota.
Personally, I have never used a purchased fertilizer on my garden. I do compost. I do add manure when I can get my hands on it. I mulch. My soil condition looks better every year and my plants don't get as big as those of my neighbors who use Miracle Grow, but they look healthy and produce well.
We need to get something straight. Plants make their own food!! Because we have been cultivating plants, growing plants outside of their natural environment and have stripped ecosystems of necessary chemicals we are now obliged to figure what is missing and what chemicals need to be replaced. This is FERTILIZER. Fertilizer IS NOT FOOD. In a normal, established ecosystem, nutrients are tied up in the vegetation. You won't find them much in the soil. Plant material dies, returns to the soil, is decomposed FEEDING the micro and macro organisms in the soil who in turn poop out chemicals and help plants assimilate these chemicals. Thinking of fertilizer as a FOOD is wrong. Too much fertilizer (and it doesn't take that much) will KILL plants! Those not familiar with botany and gardening will turn to 'feeding' their plants even more in an effort to make them healthier. I'd love to see these labels stop using the terms 'feed' and 'food'...too much like animals! Plants are not animal. That is why they are in different kingdoms!! Fertilizer should be handled with care and information about your soil's deficiencies and excesses. A soil test at least once every other year is a responsible thing to do...gardeners do get to a point where they can 'see' indicators from the plants as to what chemicals they need more of and what chemicals are in excess. We HAVE to fertilize as we've screwed up any ecosystem possible but we have to know what chemicals are necessary and what chemicals are deleterious!! Fertilizer IS NOT PLANT FOOD. Just chemicals necessary for the plant to make its own food...part of becoming 'farmers'...after hunter/gatherers. Soil tests can be done free or cheaply in the U.S. by your nearest Extension Service/University. A great source for anyone wanting to grow and understand plants!!
As I understand it, plants use the sun to make chemical energy / starch. They use the soil to get nutrients. Food for humans includes both energy and nutrients. Fertilizer is nutrients; it's not energy. So, it stands to reason that the combination of nutrients and energy is the plant equivalent of food, or at least pretty close. Fertilizer in and of itself isn't food--it's more like a multivitamin/multimineral. The soil has nutrients (and, ideally, beneficial microbes) in it, but sometimes it's deficient. Sometimes, plants can cope with this, and even breed to adapt to their environment's available nutrients (or else seeds of different kinds of plants with different requirements may colonize the area). Different kinds of plants may be beneficial to each other. However, in commercial settings, plants don't have the opportunity to do this as much, or benefit from animals as much; so, their needs are supplemented (whether with manure, compost, chemical fertilizer, whether approved for organic gardening or not, or other creative soil amendments). Nutrients from third party sources such as animal excrement and microbes should not be overlooked.