[astrobe_]

One great application I can think of is depth of field adjustment. Some games have depth of field simulation, which makes far away scenery blurry. Personally I always disable it because I think it belongs to the uncanny valley: it's nice on screenshots, but it doesn't work so well in-game because the focus doesn't change with where/what you're looking at. Eye tracking, by making this feature dynamic, would I think enhance 3D rendering for relatively cheap.

[Crespyl]

Good use of eye tracking could also allow for foveated rendering, drawing the out-of-focus areas with lower resolution and leaving more time to render the areas of interest.