[rhn_mk1]

To let it turn off when you're not using it. It uses an inertial sensor.

[JohnFen]

That doesn't explain it, though. You don't need a microcontroller for that. I suspect the real answer is "to minimize parts count".

[rhn_mk1]

Is there a fixed-function circuit that can read a digital value out of a sensor and compare it with a threshold?

I'm not dounting it's possibly, but I'd be surprised if it's available without involving a Turing-complete computer.