Commercial Art serves (unlike Fine Art, what you find in museums), you might say, a lower purpose:

Making Money.

Whether you think this is a good thing or not, if you are working in the commercial arena, there are rules. Everything you do should serve the commercial purpose, or at least not detract from it.

Of course that is often violated, as so ably dramatized by The Onion:

Graphic Designer's Judgment Clouded

That doesn't mean it can't be superb, aesthetic, or uplifting.

It means don't do things just because they are cool.