Honestly, at first I bristled at the question. "Why does everything need to have a purpose?", I mentally argued, "Can't some things just be? Just exist for the sake of existing? Art doesn't have to serve a purpose, it's just there because we create it, and we just create it because..." and then I realized that I was about to say that it inspires us or gives us meaning or, heck, even just something to look at/listen to/think about once and a while. All things that are functions, or purposes, to art.
Well, dang. I guess I can't get all up on a high horse about how art should just exist for the sake of existing - something I don't even believe anyways, so who knows why I was arguing for it.
Go watch the video on The Guardian's website here and find out what art does for us.