What's the importance of an oil pump in an engine?

Oil pump used for pumping oil from engine sump to each and every rotation parts of the engine, for reduce wearing metal to metal ,

the pump is used to pump the oil, so to lubricate the parts of the engine, it also help to cool down the rotating parts of the engine e.g crank shaft, etc

Correct answers are given. Oil is the blood for an engine

Agreed with above answers.. Without oil pump your car would not change shifts or go anywhere.. No distribution of oil to engine and can cause it to lock... 

The oil pump in an internal combustion engine circulates engine oil under pressure to the rotating bearings, the sliding pistons and the camshaft of the engine. This lubricates the bearings, allows the use of higher-capacity fluid bearings and also assists in cooling the engine.

As well as its primary purpose for lubrication, pressurized oil is increasingly used as a hydraulic fluid to power small actuators. One of the first notable uses in this way was for hydraulic tappets in camshaft and valve actuation. Increasingly common recent uses may include the tensioner for a timing belt or variators for variable valve timing systems.

Thanks for invitation 

I agree with the specialists 

Without the oil pump working the engine will seize in seconds, minutes or hours at max. 

It is like a heart of the engine,providing proper amount of lube to each part of the engine with a given pressure,If we remove the oil pump then the main bearing,connecting rod bearing,and others are seized due to metal to metal contact ,so we develop a oil film between rotating and stationary part to avoid metal contact

To pump oil all around the engine and lubricate moving parts to allow smooth movement.