Vacuum pumps are used when evacuating a refrigeration system. The pumps are needed to remove any moisture that is inside of the lines. This is accomplished by lowering the boiling point of water, thus turning it into a vapor. The vapor can then be sucked out by the pump. This is very important for the life of an air condtioner, because air and moisture condense at a different rates than the freon. These differences will cause erratic pressures in the system. Also, the moisture may erode the inside of the compressor, casuing premature failure.