Thanks for your fast answer qole and lardman
The problem is, that many people do not have skype. But they have mumble. And you can be in a channel all the time with other people who are playing games even although you are just chilling around.
So you could join a channel with some friends in it who are playing counter-strike or whatever and just want to talk without having to run skype and mumble at the same time. (and there´s a problem when you are talking with somebody in skype while you are in teamspeak, they cant hear him and wonder why you are talking )

And yeah.. sometimes... to play quake !