Whether used in print or online, designing an ad is all about one thing: delivering a message that drives a prospect to become a customer.
Most companies change their ad designs over time to reach new and changing audiences. As your company's profile changes, your market matures, and your product offerings grow, your advertising and marketing efforts should evolve to suit your needs.
You know that your audience has a finite amount of attention, so you want to make it worth their while to give you some of it. To that end, it's important that your message be succinct and simple.