That sounded a little more blunt than it was intended...
Laughter tracks 'evolved' (or perhaps rather, just came into being) because the first TV shows were, as I'm sure you know, literally just stage productions with a few cameras in between the audience and the performers. It was the only thing people knew, coming to this new medium of TV (and indeed radio before it) from music hall, and it was judged to help recreate that jovial group atmosphere. I think most people would acknowledge it's easier to laugh, and laugh harder, when in a big group who are also laughing. But, eh, reams have been written on this. I could have sworn that James Cary wrote a very good breakdown of it recently over at the 'sitcomgeek' blog, but I can't find it now...