"15 minutes before a regulation NFL game, both teams must present an "Inactive" list of 8 players to the officials. That means of a team's 53 man roster, only 45 may participate in the game itself. The lone exception is if the team has a 3rd string quarterback on the inactive list. There are separate rules for the 3rd string QB and how he may participate in a game, if at all.
The question then is, why have an inactive list? It's an attempt by the NFL to create a competitive balance on game day. Due to the brutal nature of the sport, a team can carry players on its 53 man roster that may be too injured to play in that game, but not so injured that they need to be put on Injured Reserve and miss the rest of the season.
Having an inactive list that removes 8 players on game day, basically guarantees that both teams will have the same number of healthy players available on game day, whether or not the 8 inactive players are truly injured. I do not know why 8 was chosen.
Quite naturally, the reasoning for an inactive list is controversial. Teams still must pay the inactive player their entire game day salary, and must pay to transport, feed, and lodge their inactive players on road trips. Also, limiting the amount of active players puts more players at risk of injury as it is more difficult to rest starters in blowouts, and regular players must also contribute more on special teams."