The purpose wear rings in the centrifugal pump

Wear rings stop leakage from the discharge side back to the suction side.

Because there is pressure difference, there always gonna be a tendency for the discharge fluid, which is at high pressure, to wanna go back to the suction side of the impeller, which is at lower pressure.

Wear rings are used to reduce the amount of leakage passing from the discharge side to the suction side of the pump.