A normal pump is capable of delivering more oil than an engine needs. It’s a safety measure to ensure the engine is never starved for oil. As the pump rotates, and engine speed increases, the volume of oil delivered also increases. The fixed clearances between the moving parts of the engine prevent oil escaping back to the sump, and pressure builds up in the system.
An oil pressure relief valve stops excess pressure developing. It’s like a controlled leak, releasing just enough oil back to the sump to regulate the pressure of the whole system.
