Because sometimes you need to take a knife to a gun fight. It is so you can get stealthy or quiet knife kills.
I played a game at The Mall where we were not allowed to use guns for the first few minutes, but knifes and swords were permissible. So those few players with knives were quickly able to get multiple kills, including me, before they were hosed down. Due to this experience I went and bought a flexible rubber bayonet from eBay so that I would not get caught out in the future.