The same reason why you post here - to interact with people you met on the internet and (and since its G+ and FB) to interact with people you in IRL when you are otherwise unable to interact with them due to busy lives, etc.
Anyways, for me I use both Google+ and FB to promote my videos I make rather than actual interaction with people - but I do respond to doods who want to talk to me :O.
but it's not interacting with people in the traditional sense. talking to a chat icon,just isn't the same as talking to someone face to face. It just is'nt. All social media has done is take the social out of everything. Even that no longer requires any effort anymore.