My PawPaw used it with his tomato and pepper plants. It adds calcium which helps prevent blossom end rot. You can add a handful of powdered lime, too, for the same reason. He used post hole diggers for his tomato & pepper plants, burying them until only the top four leaves showed. He also always planted by the signs--head, heart, breast, feet, etc depending on what he was planting. He was well over six feet tall and I've seen his plants head high to him, so dark green they looked black and loaded with fruit.
It's hard to argue with his methods. He never had a bad garden and Granny put up enough stuff to last all winter and give a lot away.
Seems those who are supposed to know...they have an ag. degree...say to use their expert soil analysis and commercial fertilizer, Epsom salts are a waste of time. But, and here goes the conspiracy theory, the fertilizer company also funds the university program. You can draw your own conclusions, lol. I've used both Epsom salts for magnesium and crushed egg shells for calcium with good results. My uncle would check his tomato plants and if one was looking kind of wimpy he'd lift it with a shovel and tuck a small fish or gob of fish guts and skin under it, cover the mess with a bit of dirt and set the plant back in the hole. They always perked back up and did well.