Germination rates are not always accurate, let alone in every kind of soil. Some soils are difficult to germinate seeds in, and if you just plant one seed in each hole, you may get no plants at all in any holes. If your season is short, you may not have time to try again effectively, especially where plants take weeks or months to germinate.
If you're not familiar with how well seeds germinate in your soil because it's the first time you've tried that seed source in your soil, planting lots of seeds can be helpful, for insurance, if you like insurance.
As an example, I direct-seeded lots of okra seeds per hole (four holes for each of six varieties), and I only got germination in six of the twenty-four holes (well, maybe eight or nine, but some of them died). If I had only planted one seed per hole, I might not have gotten any, statistically speaking. I also planted all the peanut seeds of two packs, and so far I only have one plant. The corn, squash and cucumbers on the other hand, had better germination. I imagine it's just a difficult soil for some plants to sprout in. There's probably a nutrient issue. Also, it's a different sort of climate. I imagine wetter, less desert-like areas would be easier for germination with direct-seeding. I could have had better rates if I started them in small containers in the greenhouse (or indoors).
Anyway, when I save seeds this year, I imagine those seeds will be more disposed to sprout in our soil than the parent seeds, which were grown elsewhere.
Now, to answer your question (because I haven't really done it, yet), there are other reasons to plant multiple plants per hole, in a variety of contexts.
- Some plants need a pollinator. Growing two or more in the same hole can potentially help with that. Some plants do better than others in this situation.
- Some plants will be smaller while growing with another plant, and still produce fruit, whereas they might be huge otherwise. Tomatillos are an example here. If you want smaller plants, this may be helpful.
- If you're breeding plants, it helps to have lots of plants. If the traits you're selecting for show through when they're planted together, this can save you space and time. There's a lot to be said for genetic diversity, even within the same variety.
- Some plants don't seem to suffer from being planted with another. However, they may end up using soil nutrients faster, and may suffer as a result at the end of the season.
In my experience, you can have at least up to three cucurbits per hole with decent results. Tomatoes usually like to have their own space, though (at their final transplant location; starting lots per container inside before the transplant and then dividing them at transplant time can work if you know what you're doing). Physalis don't seem to mind much being planted with lots of seeds per hole.