Okay, I'll explain it in a nut shell.
If you're going for young trainees, lets say 18-21, when I'm looking I won't sign a player unless he is about Superstar, or Perennial Superstar depending, the thing is, if you're looking for older players 30-31+ potential means nothing.. I'm not positive if it makes them drop attributes slower but, it doesn't much matter, they'll drop, it's inevitable, so, when you're signing younger talents, don't look so much at the stats if you want them as a project to train, make sure you want that player though, I've had a few rounds of buying a player for too much... It sucks, but it probably happens to every player. Okay, so what potential is, is how great a player CAN be, not WILL, you have to put in time to train him, you have to feed him his wheaties, treat him like your son... No, but seriously, it means a lot in younger players, not so much in older players.