I believe it comes down to when you are dead, or say in spectate, and everyone that's dead won't stfu so you can't hear what's happening in game. Not only is a nuisance, but I can see where it will be a problem when trying to administrate the server. Whether you are demoing or an admin on the server, you'll need to actually hear what's happening in game in case someone says they were free shot or freekilled. Would even apply to racism or disrespect. Say I'm demoing and someone alive starts spewing a few racist remarks but its hard to hear over everyone in dead chat talking. And backtracking to the freekill aspect, you would need to be able to hear if a CT gave a certain order or warning of some sort.
Even aside from administrating, some people still enjoy hearing what's happening in game.