Even though I am an artist myself and have taken numerous art history courses that teach me the significance of artworks; I still find myself staring at an image saying "Okay... but why?" Why is that considered art? What would posses someone to paint that? What is the purpose of that piece? From what I've read and what I've observed my best answer to the question is, "because". Because someone wanted to paint that, it's art. Because someone appreciates it, it's art. Because I said so, he said so, she said so, just because.
The purpose of art is to create some kind of effect, whether in the public eye or the artist's own.
Sometimes art is used to evoke and emotion, a love or a hatred, like or dislike, happy, sad, or confused. Other times artwork is created not for the viewer but for the artist with no intention of anything but expressing the mood or state of mind within the artist. There is often a symbiotic relationship between these two purposes. When looking at Matisse's cut-outs for instance, the viewer often gets a feeling of happiness and whimsy, reflecting that of the artist himself. Though it was not Matisse's primary intent for his artwork, and he has been stated saying that the cutouts were solely for his enjoyment, viewers can feel the joy of Matisse within themselves. Ultimately the purpose of art is to feel something, whether you are the artist or the observer determines what you feel and what effect it has on you; any effect at all though makes it art.