They are for plenty of different things depending on your position. Could be MVP in a game, a huge play or tackle, touchdowns or interceptions. Just depends on the coaching staff. Not all teams use them though as you have probably noticed. Teams like Michigan don't use them because they prefer to win as a unit rather than continuously reward the same people for individual performances. A whole lot of stickers on one player's helmet isn't so great for morale of other players who may ride the pines most games.

they get them for having a good game where they stood out. if its on a usc helmet it means they achieved the $100,000 amount in "donations" from the spoiled children alumni and backers

they have them just to be stylish i guess its just adding on to their proudness of whatever university they are.

actually they are hit marks, you get them when you do somthing good in your coaches eyes. it's the coaches decression he gives them to you when ever he wants for touchdown, sacs, or perfect attendence at practice. like i said what ever he wants.