Jambine
Full House
The main purpose of a cap is to keep most of the players on an even playing field. Same reason there is a minimum buy in.
If 8 players have $3k and one guy has $200 it is bad for the game. The short stack can go all in and will protection from the other players as the money left behind is more important than the $200.
Caps also serve a mental purpose. If a casual player is thinking about playing and sees a 2$/5 game with a max of $1,000 or a $2/5 uncapped game, the casual player is more likely to go to the capped game knowing what to expect the stacks will be like.
It is the same reason we often play $1/1 instead of $1/2. Even though the max buy in is $300 the casual player think it is only a $1 BB blind game.
Dang it ! I knew there was a reason