The ability to change the appearance of a webpage element when the mouse cursor rolls over it allows you to give visitors a full picture of what you have on offer without making them move on to a subpage.
On this sample page made using Alligator Flash Designer, the image under the main navigation menu changes when menu buttons are rolled over with the mouse. The images have been placed inside a 5-frame sprite. When the sprite zone is deactivated (i.e. when the mouse cursor is outside of it), a sheet of metal is displayed. The menu-button text changes color as soon as it is rolled over, and the image underneath changes as well.