The engine oil pressure sensor is used to measure the engine’s oil pressure. It is necessary because if the engine is run with little or no oil pressure, the moving parts within the engine would not get lubricated, and the engine would seize. Typically, the engine will not be able to start if the oil pressure sensor has become defective. Power is routed through the starter when the engine is being cranked, and once the engine starts, and the oil pressure is up, the power is routed through the oil pressure switch and sensor unit.
For more info, see How to Use an Oil Pressure Sensor