The way I see it is if you pose an interesting question or reply it starts a connection to another human being and we are a social group in it's base form so we search for each other. Then again it could be a marketing ploy or something to do population monitoring.
Weeell... People you can talk to! And basically to say that you like their answers and questions :) And so that you can see what questions they're answering and also answer them or give them a thumb! ;)