For every website we surf to, we usually see a banner or two.
Banners have different purposes for being placed on a website.
From the top, banners may look nice. Advertise Companies or sellers pay webmasters an amount of cash for their banners to be up on their pages. What drives them to do this is usually the good amount of traffic and the rich content on the site. Link Banner exchange programmes will place members' banners on pages of other members. Through this, members get traffic through the way of inter-linking.