He should also
pull a vacuum on the inside unit and lines to remove air & moisture.
If the lines are long he may have to adjust charge.


Don't these things come pre-charged and don't need evacuation?

The outside unit is precharged. The inside unit is pressurized with an
inert gas. The lines just have plastic plugs on the ends to keep out
dirt. So, the lines and indoor unit have to be evacuated first.