Getting the big picture view of a user's interest in a site helps target the sweet spot of a typical page on your site. On the web, this was most commonly done with eye tracking. The problem with eye tracking for the vast majority of web marketers was that its expense usually far outweighed its benefits.
If you could just slap your ad box on the screen like pizza-on-wall you've already won half the battle. Eye-tracking always made the challenge of finding where to slap that pizza prohibitively expensive. Because it's a highly technical, controlled method it cost quite a few greenbacks. There's a light in the tunnel now: A recent study by a Google engineer and UNC researcher found that mouse-tracking wins hands-over-fist when compared to eye-tracking.
What this may mean to you is that an incredible tool in your marketing arsenal is now possible with software, instead of the camera and infrared hardware required for most eye-tracking studies.
Eye Tracking Hardware image by MoonSoleil [cc, 2.0]