What is the purpose of placing a player on Injured Reserve in the NFL? My understanding was that you could hold an injured player out of a game and dress someone else in their place regardless of whether or not the injured player was on IR. Unless that understanding is incorrect, it seems as though it would be just as easy to continue listing the injured player as "out" for the upcoming week.
The reason for IR is to free up space on the team's 53-man roster if a player has an injury that makes him unable to play for the rest of the season.
Once a player is assigned to IR, that player's roster spot is now available on the teams roster. This roster spot is used to bring another player in to fill his space. If that player was just sat out, he would still count against the roster and the team would have no room to make an addition to fill his spot.
Once a player is put on IR, he cannot be activated again for the remainder of the season (Exception).
The point of sitting a player out is if that player has an injury that will only sideline him for 1-3 weeks or simply not the whole season. The player could also be put on the Injured Reserve/Designated To Return list.