People with mics on all the time... why?

I don't play multiplayer games usually, but man I'm so weirded out by this. Have muted so many players.

Nothing wrong with making some fun commentary or asking/saying something once in a while, but why on earth does everyone need to hear you whistle, sniff your noise, hear the traffic outside of your appartement and all that nonsense the entire game?

What's the point..? Some I've heard is downright nasty or insanely loud. I don't get it.