Obviously, there's the benefit of getting valuable experience. But, FWIW, there's also a financial benefit to them, as well. Each year, the NFL has a "performance-based pool" that rewards every player who gets on the field. The more you play, and the less you earn, the more the pool pays at the end of the season. And, as I said, every player who gets on the field for a down gets a check: Tom Brady got a check for $121 last year, before taxes. Gary Guyton was the biggest rookie beneficiary last year: for playing over 30% of the team's defensive snaps last year, at a rookie minimum salary, he got a bonus of $213,128, second only to Matt Cassel, who played nearly every O down on a salary cap figure of about $500K. So players like Julian Edelman, Myron Pryor, and Isaiah Stanback are earning something on the order of $300+ for every down they're on the field.