the screw have 2 applications
1 to hold the ring in place after you adjust the TDC
2 to hold the sensor in place as it can move FW/BRW over the magnet
3 the ring is held by an C clip ring as well - look at the photo you will see it

the screw isnt 90 dgree to the shaft, it is in an angle so it put pressure on the sensor housing as well

gadi