We see articles and advice all the time about the importance of optimising your website; articles about how tweaking very small things from a usability standpoint will help increase your conversion rate by 10, 50, or 200%.
Well, in media we can consider the banner to be a type of microsite. If we do that, then tweaking and optimising your banner becomes a very important factor in delivering on your targets for your display campaign. The article, Optimizing Creative: Testing Benefits and Offers, discusses the very basics of getting your ads right, showing that about 70% of the ad's influence comes from the ad's benefit and call to action.
These are the two elements that should be tested. We always recommend using at least 2-3 creative variants in our campaigns and optimise to the best performing, of course. Tweaking your ad to increase your CTR from 0.05% to 0.08% on a ten million impression campaign is a difference of 3,000 more clicks to your website!