I thought that the purpose of art was as a distraction. Everybody in society needs nice things to do, see, hear or feel every now and then so as not to get too bogged down in work, family, friends, country, religion or whatever.
Art is not like science. It does not build on prior findings/discoveries (prior art?) very well. Nonetheless, art is probably vital to a healthy society.
Update (the day later) - I'll have to restate myself here. Art does not build on prior art very well. True, you have new trends in language, development of perspective and so on ... but seriously ...
It's not the same as in science. You build on previous knowledge. Computing came out of logic and maths and physics, genetics grew out of biology and maths ...
What do I see in art? Revolutions in what? Post-modernism? Abstract art? Cubism? The best things I see in development of art tend to be in language, otherwise it's just new ways of painting or making sculptures or displaying things. The medium has changed (thanks to scientific discoveries) but has it changed the nature of art? Artists are still trying to display the human spirit or human emotions or human bodies or the environment with whatever medium they have available. This has not changed in thousands of years. Science, on the other hand, advances differently because it takes prior knowledge and tries to extend it.
Design is a subset of art, but isn't exactly equal to art. Design tends to build on prior knowledge a lot more than plain art for art's sake. Building design is a whole different kettle of fish than paintings.
Finally, danlowlite - great art has a great capacity to distract. The Mona Lisa will distract people for many generations to come ...