Just a guess, as I only know a little about hearing aids, but maybe it's to measure the acoustic properties of the ear canal to be able to dynamically reproject the frequency spectrum of incoming sound to bands that the ear in question can actually hear.
Yes. There is research dealing e.g. with ear canal-based authentication. Other earables (Omnibuds by Nokia Bell Labs http://omnibuds.tech/) use the inward-facing microphone to detect the sound of the heartbeat. With this (plus a PPG sensor) they can estimate blood pressure (after calibration).