[HALtheWise]

The obvious reason for it existing is to serve as a glass breaking alarm, something that might have been part of the planned specs, and listed on the spec sheet, when they finalized the hardware design but they couldn't get working for launch. That's a very real feature people want their security systems to have, and forgetting to add "a disabled microphone, IMU, bluetooth and FM radios, and probably some other random chips privacy-obsessed people will care about" back onto the spec sheet is a really easy mistake to make, especially a few years ago when Nest mostly had user trust.

To be clear, I don't know exactly what chips modern electronics contain that we will worry about in a couple years, but all of the above seem like plausible things.