PerfInit said why - "It is a backup to the mechanical fuel pump and also provides more positive fuel flow during takeoff, landing and switching fuel tanks." I would add for starting too.

Fuel in low wing airplanes isn't going to move to the engine on its own. An electric pump provides redundancy and fuel under pressure when the mechanical pump isn't working. High wing airplanes use gravity to move fuel to the engine. The odds of gravity failing on any given flight is low enough there is no need for backup.