The back screw is a for a certain special kind of mounting point in some cars

Like in the attached picture. Rubber knob mounts on stud screwed into cradle,
knob fits into hole at the centre back of the DIN opening. My Polo has a double DIN opening, empeg mounted at the top, you can see the hole at the back of the lower DIN opening. Of course, an ordinary back strap (perforated metal strip) can be attached if no such special arrangements exist in the car.

/Michael