What is the function of the overflow pipe ?

The overflow pipe serves two functions:

First, When the water seal in the toilet bowl is insufficient, the refill tube of the inlet valve directs water through the overflow pipe to replenish the bowl. This helps form a water seal in the U-shaped trap, preventing sewer odors from escaping and contaminating the bathroom.

Second, In the event of a severe malfunction or rupture of the inlet valve that prevents it from stopping the water flow, the overflow pipe allows excess water in the tank to drain out. This prevents the tank from overflowing, which could otherwise flood the bathroom or even adjacent rooms, causing significant damage.