This will allow for some sites to have a call-to-action message that will act like an advertisement. Others can make their header look similar to a Twitter page.
There are several styles:
1. just your logo and header text
2. header image plus text plus overlaid logo
3. header image plus text
4. just your ad-like message (image), no header text

A video is available to show you how to make the changes.
Video
Some sites to view:
Video explicativo: http://youtu.be/y6DDFnjZd1U