When the engine is in the car and you remove the oil pan, it is a good thing when the oil pump comes down inside the pan. The pump retaining screw is a must use when installing the oil pump and pan.
Many years ago, I had an engine rebuilt by a shop, and the person who assembled the engine tied the pump into place with cotton clothesline cord. The cord was left in the engine. The cord disintegrated, it clogged the pump screen and the engine starved for oil while away on a cruise.