My guess it is for the security system.
Detachable Face Security: This unit features a motorized "flap-face" front panel with Detachable Face Security. If the front panel is not detached from the head unit within five seconds of turning off the ignition, a warning tone will sound, and the face will fold down. The warning tone can be turned off in the menu, if desired. A face-fixing screw is supplied for making the face non-detachable, if desired. A hard plastic face case is included with this unit.
but like i said...just a guess.
