Hello
From what I understand it shall detect any hole just in front (or in the back) of the robot and allows to stop the mower before it tries to mow the bottom of the swimming pool !!
Seriously, detect hole and avoid dropping in. I guess you may need it if you do not have a perimeter wire. It is mounted as much as possible in the front of the mower to have enough time to stop (same on the back when driving backward).
Jacques