Yep, that's another great question. Personally, I like the idea that art is any form of human expression that exists for its own sake. Not in order to be instructive or useful or to make money but simply because the person creating it felt like it (obviously this is an ideal and real life motivations vary).

More pragmatically, one might ask what art is good for, but since you didn't, I'm not going to ramble here.

That said, there is the question I raised in my comment whether the work needs an audience, someone to behold it, in order to fully become art. I believe it does. If you paint a picture in the dark and hide it so nobody ever sees it, I struggle to accept it as art.

What's your take on these questions?