Besides the tangible benefits others have mentioned, I think everyone falls prey to the desire to be "well liked". Karma means you wrote something other people liked or appreciated, so it is gratifying to watch your karma go up (because that means people like what you are saying).

Silly I know, but I also recognize that it is what made me try to write good comments when I first joined HN. Which is exactly what karma should do - make you try to be a better poster.