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. Tell or Inform Certain banners inform the website's audience about an event or activity. Most of the time, this event or activity is an important one and may usually concern many, including the visitors.
Some websites have banners to fill up an unoccupied space on their pages.
This case is mostly very rare and minimal. Tell them Clearly The message to your visitors should be clear and consise. The font should be of the right size, clear, easily seen and easily read. This will ensure that the visitors get the message quickly. Like above, your visitors need to get your message clearly.