You can pour it on top. (Gravy.) You can cook one or more components in it, for the flavor. (Spaghetti and meatballs, macaroni and cheese.) You can use it to hold everything together. (Mayonnaise in countless salads.) Sauces add seasoning to bland foods, moisture to dry foods, and interest to everything.
A very quick and easy one I learned many years ago is a simple peanut sauce. This was making the rounds of vegetarian sources back in the 80s - inspired by an Indonesian dish, gado-gado. Well - most books called it gado-gado, but it isn't, really... (With the wonders of the internet, you can look up real gado-gado. It looks delicious - but it's not this quick and mindless...)
That said - here is a simple vegetarian dish made interesting by an easy sauce, which, several levels removed, was vaguely inspired by an Indonesian food...
I used tofu. Chicken works well, too, or, really, almost any meat or fish I can think of. Tempeh has the advantage of being, even... Indonesian! The basic meal is some kind of protein, with some kind of vegetable, cooked in, or served under, a peanut sauce, the whole thing served over rice. In this case, I cooked the sauce separately from everything else, and poured it over.
For this, I prefer a firm tofu. For years I read, and ignored, instructions to put slabs of tofu on a slanted board with weights on it to make it firmer. Do it if you want - too much fuss for me... Then I learned a method of simply wrapping it in a clean kitchen towel (or even paper towels, if you must, but the paper gets soggy) and letting it sit on the drain board while you chop onions or start the rice or whatever your first step in cooking is. I find that just that little bit of time and attention makes a big difference in the end product - it's firmer, and browns nicely, and seems to get a bit more flavor from whatever it is cooked with. (If you prefer a soft tofu, don't do this - that's a different method, I'll talk about it another time.)
So, I wrapped my tofu and set it aside. Then, I sauteed an onion in a little olive oil, and added a bit of the chopped hot pepper I froze. I let them cook until just soft. Then I unwrapped my tofu, diced it, and added it to the pan.
If you use a stainless steel or enameled pan, the tofu will stick, at first. If you try to stir it too soon, the cooked browned bits will peel off the cubes, and it's messy looking (though fine to eat.) It feels counter-intuitive - but if it sticks, leave it for a while longer. When the side touching the pan becomes firm and golden, it will magically release, and you get the nice cubes (or slabs) of browned tofu. *Then* stir it around, so the other sides have a chance to do the same - and the vegetables also move around and soften.
Meanwhile - I heated brown rice, and steamed broccoli... (If I'd been cooking the rice fresh, I'd have started it first.)
Once the tofu and aromatics were nicely cooked, I put them over rice on the plates, and arranged the broccoli around it. Then I started the peanut sauce (Yes - it's that fast...)
I used natural peanut butter - just peanuts and salt, thank you... I scooped up a big spoonful for each of us - a heaping tablespoon per person, basically - and put it in the pan with about a cup of water, and a splash of soy sauce. (OK - true confession - this was more than a splash. There is something wrong with the plastic insert in my soy sauce bottle... This was Way Too Much soy sauce. Tasted OK, but... not Great... saltier than we'd usually want... This means that, when you do it, the sauce will have a lighter peanut color. Don't be surprised.) Normally, at this point, I'd also add some Tabasco or other hot sauce, but this time I'd put hot peppers with the tofu, so I skipped it.
I heated the mixture, stirring. (I use a silicone spatula - it's great for scraping the pan.) At first, it looks pretty awful, with blobs of PB in the liquid, but it emulsifies quickly, and then thickens surprisingly. You may even find that you want to add more water, in a bit, if it gets too thick. Once it is heated through, and smooth, just pour it over the rest of the food. For family service, it can be put in a sauceboat on the table - let everyone help themselves.
I did this separately, so you could see how the sauce itself works, and that it can be used in many ways. I often, though, just add the water and peanut butter to the tofu or chicken or whatever in the pan, and go from there. I may even have all the vegetables in that mix, as well. That probably works better for just one or two, though.
Per Serving -
1/2 c water
1 1/2 T natural peanut butter
1/2 t soy sauce
dash hot sauce - opt.
Put all ingredients - using roughly those amounts per person - into a pan over medium heat. Stir while heating, until sauce becomes smooth and thickens. Add more water, if needed.