Originally Posted by PerfInit

The Primer is used during cold engine starts only. The primer manually Injects a small amount of fuel directly into the cylinder head of a carburated engine. Think of the primer as a small “syringe” with a tube that goes directly to one cylinder. The electric fuel pump is for when the engine is running. It is a backup to the mechanical fuel pump and also provides more positive fuel flow during takeoff, landing and switching fuel tanks.