It's the only way to do it. The light sensor delivers a realtime feed of what your eye currently sees. A website can react appropriately.
It's actually more powerful than monitor calibration. Calibration doesn't account for the viewing condition (the brightness of your environment) whereas this hardware can.
It's a device, so it can feed the information to all the programs on your computer. WebUSB is a way to make it useful from day one, but it's not the sole target.
WebUSB is a way to make it useful from day one => if WebUSB was marketed with this business goal, then why not (except nobody would buy it)? but the implications are so much wider, in terms of security and usability...
Why not WebSATA? WebPCI?
There are certainly a few key actors who would benefit from such a standard (benefits could be time to market), I'm just not sure it's enough benefits for the end user to justify a new standard!
It's actually more powerful than monitor calibration. Calibration doesn't account for the viewing condition (the brightness of your environment) whereas this hardware can.