I wonder if the accelerometer is to pair with the light sensors to handle light angle to adjust truetone maybe? (I legit can't think of any major uses of an accelerometer in a macbook air). Other thought is that the same chipset is going to be used on a future iPad and they just included it and disabled it because a single production line is cheaper and easier to manage.
EDIT: someone suggested spatial audio just below, that also makes sense, more so than my suggestions given the screen can move without your head moving on a mobile device.