The oil pump pressurizes oil to be pumped throughout the engine. A pressure relief valve is built into the oil pump. When oil pressure exceeds a specified value, the valve opens, allowing oil to return to the sump or pump inlet.
A stuck-closed pressure relief valve can raise oil pressure to dangerous levels. This extreme pressure can cause the oil filter to explode, resulting in engine oil loss. Excessive oil pressure can also cause pressed-in oil galley plugs to blow out. Either of these scenarios can result in complete loss of oil pressure and catastrophic engine failure.
The moral of the story is: too much oil pressure can be just as dangerous as too little. A properly functioning oil pump pressure relief valve is critical to engine health.