I thin most of my fruits, and some of my vegetables. It makes a definite difference. Most of my fruit trees are not old enough to be heavily-bearing yet, but even in cases of only a few fruits it is helpful for larger, healthier fruit. Producing so much at one time is very taxing on a tree, and while usually they are able to make a selection themselves of who survives when resources are scarce, when they are getting special attention like extra water or nutrients, they can't really make a good choice and put energy all over the place. If you prefer quantity over quality, that might be perfectly fine, or if you just like small fruits. But I have noticed that the more I thin out fruit, the better the resulting ones taste, and the larger they are.
Some of the most striking changes I have seen in loquat, and in barbados cherry. The loquat resulted in fruit twice the size, after thinning by half. In the barbados cherry, the change was amazing - after both pruning the shrubs of several significant limbs (I am shaping them to appear like trees rather than shrub-like), and cutting back unnecessary fruit, the leaves themselves became twice the size, and the shrubs started pushing flowers profusely. The fruit did not come out that much bigger, but were tastier, and the plants kept trying to put out more fruit the more I would remove. So thinning definitely does make a difference for plants. In tomatoes and peppers, I have seen that thinning results in bigger and often stronger-tasting fruit. It is similar for types of lettuces, as well as for herbs - if you trim back some leaves, the rest get much bigger, tend to have deeper color, and better taste. But there's less, so it's a trade-off of course.
I did the same with mangoes, but at this time they are still small, so I only have limited personal experience of the effects. In this year, as soon as I cut back fruits (I started with about 50), the remaining ones got quickly much larger, because all of the plant's energy could go into fewer fruits. I cut them down to 7, and ultimately just 2 mangoes, which so far look very healthy. Naturally it's not necessary to be this drastic with an older tree, but since this is the first year of fruiting many would argue it's best to remove all fruit entirely so the tree can better establish. And I would... but I don't want to wait a whole other year to try the first mangoes!
I have a Hasya Sapodilla, and this is its first year to fruit. It has put out what appears to be over 100 fruits! There is no way this young tree can support all that, so if it does not weed almost all of them out soon, I will have to do it for it. I haven't seen the result yet, but I would assume it would be the same, larger and healthier fruit. The tree has been ridiculously healthy despite almost no care, and I am not obsessive about sapodillas, so I probably won't thin this one as much as mangoes - I might leave as many as 15 fruit on it this year if the tree looks like it can support the weight and nutritional needs. If it's struggling, I'll twist nearly all of them off.