I'm intrigued by it. To be clear, it reads surface temperature, not ambient temperature. The demo gave the example of a hot pot on the stove, I think, and they mentioned they've submitted something to the FDA to take your temperature (to see if you have a fever).
Seems to me to be a holdover from 2020. Interesting that Google assumed we'd still be measuring temps at this point.
This was my first thought as well. Curious to see if it functions similarly or within a certain tolerance of one of those much more expensive FLIR temp gauges you see laptop reviewers use. MKBHD video showed that it asks for 5cm distance from object being measured and for you to tap to select type of material, which notably excludes skin temp.
It may exclude skin temperature but during the keynote they mentioned working the FDA to allow it to be used on skin.
Maybe part of face unlock like liveness detectors for fingerprint scanners.
Maybe the plan is to eventually have the phone include every possible sensor and do everything.