The booster has these ramps that push to each other as the clutch tries to slip
The one part climbs on the other and they lock causing higher pressure to the clutch discs against the pressure plate.
The grove reduces the force from the belevil spring, because it loses some tenths of a mm, as it rests in the groove. So its function unsticking the clutch discs is compromised. The grooved disc needs to be replaced with another one.