Visual impact is a huge factor in every form of marketing other than radio.
There's a fundamental at work here:
The first barrier every marketing piece has to overcome is getting noticed at all!
I know I've said this many times, but it remains a central factor in all marketing. If someone doesn't stop and look at your ad, website, mail piece or commercial, you never get a chance to get your message across. It doesn't matter how brilliant your pitch is if it is never heard or seen.
And it isn't words that make people stop and look. FIRST they have to stop. THEN they'll see your words.
It's the appearance of the ad - size, placement, colors, images and their arrangement - that impinges and make people stop.
This is the first consideration in design.