I honestly feel like the roster thing is stupid, all 55 should be able to suit up every game. Only the Practice Squad should be barred from playing on game days.
It's an attempt at making the game more-fair.
Some of your players are going to be hurt some weeks with minor dings and pulls. By keeping the game-day actives smaller than your total roster, you can sit a couple of people and not be at a disadvantage vs. a team with everyone healthy.
There is also the concern that with 55 game day rosters, we'd end up with even more hyper-specialisation than we already do. 55 on game day isn't enough yet, but could you imagine having three long snappers? A left-hash, right-hash, and center of the field long snappers? This is an admittedly comical example, but this is the kind of situation the NFL is trying to prevent.
