In order for the refrigeration process to work efficiently (and at all )the condenser must be able to dissipate heat. If it can't, the heat travels back to the compressor and will blow an overload and a start relay (a $150 to $250 fix). The condenser fan pulls hot air from the condenser and compressor area to cool them and blows it across the evaporator tray to evaporate the water in the defrost tray.
THE FAN ALSO PULLS IN DIRT, DUST AND ESPECIALLY PET HAIR THAT COVERS THE CONDENSER.
Regular cleaning of the front won't allow dust to migrate to the back where the fan pulls it. If the condenser is a tiered style a refrigerator brush MUST be used to clean. If an angle style the best way to clean is with a vacuum hose. DON'T USE FORCE as the condenser has freon going through it and is sealed. Read the owner's manual for maintenance instructions.