Design is the art of planning, and it is the art of making things possible.
Having no purpose is the function of art, so somebody else can look at it and ask a question. Design is different - you're supposed to understand what's going on. You can be delighted by it, intrigued by it, but you're supposed to know it's a hot dog stand.
Art has no purpose. It exists for its own sake.
Design always has a purpose, art has no purpose. That's really the difference between them. Do I think one is better than the other? Absolutely not. I think they both fulfill functions.
I think design, to a degree, is more generous and more humanistic than art, though great art can move us more.