A play is one of the fundamental statistics when analyzing a player's performance. A play starts the moment a ball is snapped and goes until the officials call the play dead or someone scores. Everything that happens between those two points can be recorded and analyzed on a per play basis.
Comparing statistics on a per play basis is especially helpful for teams trying to analyze players with different amounts of playing time, or who play only sparingly, like special teams players. If a player in college is second or third on the depth chart, they may not be on the field for many plays. But if a team is able to evaluate exactly how well the player does when they are in the game, on a per play basis, they can extrapolate his performance to see how he would do if he received more playing time.