Salvador Dali wrote that "to gaze is to think". He would've made a great advertising art director. We need all the thinkers we can get.
With thoughtful art direction, an ad is easier to read than pass up. Without good art direction, an ad is easier to forget, not take seriously, and it's easier to mis-judge the quality of the product.
Just compare an Apple ad to any ad for HP. Which one reeks of confidence and timeliness? Which one makes you either crave to own the product or proud you already do?
Some ad agencies are better at this than most. Chiat/Day, Apple's agency, rarely fails to captivate. The art direction reflects the passion in the product. Anything extraneous is thrown overboard.
Doyle Dane Bernbach turned the little VW Beetle into a gem of engineering (and attitude). The same agency's work for Sony made us sit up and respect Japanese ingenuity.
I've always sought out art directors who knew how to go for the heart or the throat. You can love a person into reading your ad or you can scare her. But if the art direction is wishy-washy, the reader will be too.
We go through life uttering phrases that are born of our dependence on what we see. "Seeing is believing." "Out of sight, out of mind." "Oh, I can see your point of view."
If we aren't grabbed by an ad or a commercial, we won't engage with it --- no matter how beautiful the words. Anything less than great art direction is invisible.
My motto is "think visually first". I've learned that the hard way. Which is why Dali is a master.