depends on his diet, pretty much anything we give them that isnt grass is a supplement. What is needed and isnt needed depends on the grass it's on. Supplements like electrolytes, vitamin A, Biotin, etc etc are things crucial to a horses well being but are generally lacking at least part of the year every where.
However with that said. Just like the human supplement market, the vast majority of what is out there is total snakeoil, and basically does nothing but get you to spend money. Alot of operates on the placebo effect if it does anything at all.
Generally quality feeds tend to have most compunds needed but we ususally don't feed the recommended amount especially if we have plenty of grass, we'd end up with lardbutt horses, I do a basic pasture supplement which contains biotin and Vitamin A among other things.