Well worth doing. Lots of advantages if done carefully, although obviously not taken to extremes.
1. Overloaded branches can bend or break damaging the tree.
2. It avoids a crop made up of just very tiny apples.
3. Allowing an excessively heavy crop to mature can trigger an apple tree to a biennial fruiting pattern, so is best avoided.
4. If you inspect the tiny apples you will see a proportion that already carry a blemish or imperfection. These will never produce good mature apples so eliminate them early and let the tree put its energy into the best.
5. Same argument applies to small apples that are poorly positioned under foliage so will never get much sunshine and will not ripen to their best.
I have a few apples (such as the lovely Pitmaston Pineapple) that will always produce clusters of small fruit (or rather even smaller ones in the case of PP).
One of the things I've noticed is that the point where they touch is a favourite point-of-entry for maggots (presumably they see them as a place to hide).
By thinning in June I try to reduce these points of contact by making sure that when I leave two or more apples in a cluster I've tried to leave the ones that are furthest apart. On PP I tend to go for one per cluster unless clusters are
very few - on an old tree it's the only way to get the "standard" size ones.
If any touch later on as they swell I check them again and remove whichever apple looks worst hit - or both if there's a third unblemished one.
It's a bit of extra work but nothing is worse than finding two otherwise perfect fruit with a single hole going through both.
Another reason for only growing varieties that are better than anything in the shops - otherwise what's the point?
Anyone who hasn't got Ashmeads Kernel, PP and William Crump (the only absolutely first rate red one) is missing out (and I could recommend a dozen others that are better than "peoples favourites" like James Grieve - it's just barely OK).
IMHO anyone who wastes their time and effort growing Gala or Elstar needs their head examining (the mushy sweetness, that bitter skin) - though it's understandable if they were brought up to prefer a supermarket apple pie to a good fresh apple...
Cheers.
PS. Policymakers at Brogdale please take note of the above - I and many of my friends stopped making that long trip to visit when the numbers of specialist fruit on offer for tasting and buying started to be swamped by horrible varieties like Gala or Elstar that presumably grow like weeds.