A couple of reasons. Eye trackers are an incredibly common tool for research. They're obviously useful for studying eye movements themselves. This may sound ni

A couple of reasons. Eye trackers are an incredibly common tool for research. They're obviously useful for studying eye movements themselves. This may sound niche, but eye movements are tightly integrated with perception and attention. Eye tracking can thus tell you about the features or cues that help (say) hit a baseball, read some text, or find a tumor. Experts in a field often make different patterns of eye movements than novices, so there are some nascent efforts at using eye tracking to help training (e.g., "no, keep your eye on the ball"). On the application side, they have potential as an interface device too: you can make eye movements incredibly quickly (80-100ms, under some conditions) and accurately. This persists even in some neurodegenerative conditions. Foveated rendering is another neat idea. The fovea, the "high resolution" part of your retina, is surprisingly small: it captures a thumbnail (at arms' length) sized region of the world around the point of gaze in color and detail. Outside that region, the visual input is increasingly blurry and colorless. It doesn't seem that way because your brain assembles percepts by making many eye movements (2-4 per second) stitching the input together. One popular idea is to exploit this when rendering images (e.g., for VR): spend most of your cycles making the point of gaze pretty and provide just enough elsewhere to complete the illusion. Doing this obviously requires knowing exactly where a person is looking, and thus, an eye tracker.