Face tracking and eye tracking allows them to determine your point of focus on screen (known as "gaze tracking") as well as how you are reacting to what you're seeing via mood/emotion etc. In the context of the Quest, this gives them a metric of how much you're enjoying certain games, which would allow them to give you more accurate recommendations (ads) for new ones that you might like to buy. Height data can be used to target kids with a very high degree of accuracy, even if they aren't using their own account. It's not about selling these individual pieces of data, it's about more accurately sorting you into the demographics that they allow advertisers to choose from (and potentially open up new ones). Because accuracy sells - if your ad company gets the reputation of being inaccurate, you will lose customers.

It also gives them information on how much you're paying attention to the advertisements you're being shown. One of the big questions in advertising is always "how do consumers respond to our adverts" - how long are they looking at the ad etc. Gaze tracking gives them that information, and has been an area of research in the advertising field for many years. If you're someone who pays more attention to adverts, then you're a more valuable target for advertisers, especially if they are able to make the link between you seeing an advert and a purchase being made. Or if you're someone who doesn't particularly pay attention to static ads, but will pay attention to a video ad, Meta can use that information to tailor what ads you get served. Conversely, if more people than normal are not paying attention to a particular ad, then Meta can more accurately report to the advertiser that their ad is performing poorly, likely far sooner than with other metrics.

So by doing this, Facebook/Meta has improved on two of the core parts of advertising: accuracy and retention. If you are able to boast to advertisers that you can target certain demographics with a higher degree of accuracy, then not only will more advertisers be interested in your services, but you are also able to charge them more money for that luxury. Similarly, being able to boast a higher viewer retention is of great value to advertisers, especially if this can be linked to higher sales, as is more accurate information on how ad campaigns are being recieved.