Imagine, for example, opening your favorite brand’s website and being presented with a miniature virtual storefront. You could look at their most popular products as if you were standing on a sidewalk peering into their shop. I imagined this as instructed as was simultaneously bored and depressed. Why does it always have to be about shopping.
It's a relatively invasive technique that requires a computer's camera to record details of a person's face, and that's marketable data that any commercial shopping site would likely collect and sell on to other parties. It'd be much simpler to implement arrow key / spacebar navigation through a 3D scene (still computationally expensive relative to rendering the standard HTML/CSS/JS page), but then that data couldn't…
It wouldn't be so bad if it were a browser feature, i.e. a website can use an "Eye Tracking" API instead of the Camera API (with permission granted), and get limited data.