"[Design is] the synthesis of form and content. In other words, without content, there is no form. And without form there's no content. A work of art is realized when form and content are indistinguishable. When form predominates, meaning is blunted. But when content predominates, interest lags. But the genius comes in when both of these things fuse." -Paul Rand
My favorite quote explaining why art is important comes in Kurt Vonnegut's book Mother Night.
Acutally, I suppose this is only a motivating quote if you're concerned about legacy. I don't think that I am...
"Howard-" he said to me, "future civilizations better civilizations than this one, are going to judge all men by the extent to which they've been artists. You and I, if some future archaeologist finds our works miraculously preserved in some city dump, will be judged by the quality of our creations. Nothing else about us will matter."