If you don’t have the service equipment to put the repaired system under vaccum in excess of 28 in/hg then water vapor can remain in the system and damage the compressor. Also you need the right amount of refrigerant for optimal performance. It really depends on what your doing, but I see people come in once every other week or so during the summer with the “I just put a can of refrigerant in from autozone and it still doesn’t work” line. You evac the system and find 2 lbs too much R134 so the high pressure switch trips disabling the system.
windshield replacements sure are a bitch. Ive done a few successfully but also broke a few. Very easy to break the good one when removing it from another car.