What is the use of the ring on the racing chair cushion?

The plastic buckle is used to fix the cushion

There are generally two snaps in the back of the seat against the back of the seat. This snap is inserted through the gap between the seat and the back until it is withdrawn from the back of the seat back and then it is clamped laterally between the backrest and the seat. It is used to fix the cushion not to slide forward. Some seat cushions also have circular buckles on the sides.