The purpose of mulch is to keep weeds down, feed the organisms of the soil, retain moisture and improve whatever soil it is that you have. Ideally, it should look beautiful. Bare soil is not a natural thing, but covered with a great mulch it'll be beneficial to the ecosystem of which even you are a part of and your plants will not flourish without.
Un-decomposed mulch will not feed your soil organisms. Bark and chips and plastic are un-decomposed mulch. They help with moisture retention, not so much with weed suppression. Soil organisms are critical to the health of plants. A good soil is full of organisms both micro and macro. Bacteria is an example of micro organisms, earth worms macro. Without a thriving community in your soil your plants will suffer. They will survive but continue to die slowly. Plants and soil organisms are absolutely necessary for each to survive.
One of the best things you can do for your landscape is to use DECOMPOSED organic mulch. If you make your own, you can probably trust the product. Decomposed organic mulch (not rock, not bark, chips and certainly not plastic) is what feeds both micro and macro organisms in your soil. It is like putting up big neon sign saying, "Great Eats Right Here"...They'll come to you and they'll reproduce...these organisms come up out of the soil and eat decomposed organic material, return into your soil profile, poop the stuff out and in doing so they are aerating your soil, mixing organic matter into your soil and improve your soil. More importantly there is an important symbiotic relationship between your plants and these organisms. The plants actually 'pay' or provide these organisms with up to 40% of the carbon they produce. The organisms do mining, aeration and get nutrients to the plant roots so the plants get the chemicals they need to make their own food. With which they give back to these organisms.
Without decomposed mulch and small amounts of added nutrients, this process takes a nosedive. Urban areas and suburbs are becoming deserts because of the use of bark and plastic. There is nothing for birds and beneficial insects to eat. The cycle is stopped. Your plants will linger, your soil will become compacted and it will take more water to reach the roots.
Most people are unable to see the difference.
The bad news is that a source of dependable decomposed mulch is hard to find. Most are full of pesticides from homeowner use and weed seeds. There is one I have found that I trust and am trying to get even experienced gardeners to try and that is decomposed human poop mixed with sawdust and professionally decomposed. The final product is NOTHING like the original ingredients. No smell, no sticks, no rocks, no weed seeds, no pesticides and it is beautiful! Dark taupe in color (not black, how natural is that?) and fine, fine textured, smells like earth as it should. It is also tested, 5X!! A reputable place will be able to give you a current test to show exactly what it is composed of. No other mulch that I am aware is like this.
The companies producing this stuff can deliver it, you can pick it up in a truck or they can even blow it onto your beds or lawn. It is not expensive but whatever the cost it is the cheapest, most intelligent way to care for your soil and plants. Install 2" (I might go and chop down big weeds but I like to reserve my energies and body) right on top of all bare soil and weeds. No weed seeds are included and the multitudes of weed seeds in your soil will never be able to germinate. Any weeds that do make it through are easy to pull up.
This stuff will be promptly eaten and mixed into your soil by the very happy organisms. This helps to stabilize pH, adds nitrogen, helps retain moisture. Within a week, I am so very serious, you will see your plants start looking like they should, greener, stronger growth, more able to deal with disease/insects.
I've had blue clay and inside of one season watched as black streaks colored the blue with organic matter and air. All I had to do was put a couple of inches of this mulch on top of the soil.
Besides tasks like mowing correctly and at the correct height (PNW grasses no shorter ever than 3"), aerating your lawn, fertilizing properly for lawns and plants (under fertilizing is better than over fertilizing...well, over fertilizing will make the plants reduce their carbon donations to the organisms because they are getting more than enough nutrients and a little more fertilizer than that will KILL your plants), watering deeply after allowing your soil to dry out (to train your plants roots to grow deep and become drought tolerant)...the best practice one can do is to mulch your soil with decomposed organic mulch. This aerates your soil (plant roots need air), adds organic matter into the soil which stabilizes the pH, adds nutrients, helps to retain moisture, nutrients aren't leached away into the water table, keeps roots protected from extremes of temperature, feeds the organisms plants need to uptake nutrients and to bring them nutrients, stop germination of weed seeds that would compete with nutrients and water, your plants will be able to fight disease and insects that come their way, they will be vigorous and beautiful, your soil whether clay or sand will continue to get better and better...all you have to do is replace this mulch as it does disappear into your soil and cause all this other stuff to get done without anymore effort on your part. And every other year is what I am used to for myself and my clients. If your soil is particularly poor, you might have to replace it the next year after the initial coat as the starving organisms gobble it up and drag it into your deficient soil. This only shows your soil is coming back to life, the mulch is being incorporated into your soil and not being washed away! If it stays, your soil was in a better condition. It uses what it needs.
This one practice, feeding your soil with decomposed organic mulch by just dumping it on top solves so many other problems it is worth the little time, effort and money invested. It is like killing 4 birds with one stone.
Call your sewage treatment plant to see who and where they produce this stuff. Up in the Seattle area it is called Gro-Co.
There is only one down side. It has higher levels of heavy metals (from our medications) so you shouldn't use them on vegetable gardens. I won't mention that most of the world outside the U.S. uses human manure for most of their agricultural production. Yet these other countries won't accept our GMO crops...A lot of our tap water has higher levels of heavy metals than does this stuff. I still don't use it in my vegetable garden...but tempted.
It is tough to change your paradigm when so many people use bark and plastic for mulch. It is simply wrong and doesn't reduce weeds...it makes soils that weeds themselves chose not to grow in. This answer was long, I apologize. Even people that are well-educated and experienced gardeners have a hard time not using the oh so popular bark and plastics.
If you do try to find this stuff in your area and do your own research...and try using this, please let us know if you could tell the difference, how it was with weeds, could you tell the difference in your landscape plants and how long it took to see how much more beautiful it was than bark. Finer texture and a more dirt-like color should really highlight your plants versus detract. Did you notice more birds and wildlife? We humans do best by copying nature to get what we want instead of trying to force natural things to do our bidding. Bark will eventually be in the same category as astroturf and polyester. Grin. I truly hope this makes sense to you.