[Porsche944]

Well, you're close.

The part about Oxygen causing corrosion is true, as is the part about removing moisture (water).

However, the main reason for evacuating an A/C system is to remove ALL non-condensable gasses AND moisture.

What you have to keep in mind that the system is designed around the heat transfer characteristics and physical properties of a specific gass.

If you introduce any "contaminants" into the system you modify how the refrigerant behaves. For example, adding salt to fresh water actually lowers its freezing temperature, and raises its boiling temperature. This is why some of the first refrigeration units used brine as a refrigerant.

Having moisture in the system presensts another problems.

First of all, water does not compress, so any significant amount of water in the system could damage the compressor.

Secondly, water turns into a solid at low temperature, and in an A/C system, ice will form within the expansion valve (or orfice tube) effectively blocking refrigerant flow.

Another benefit of evacuating the system is leak detection. Any A/C tech worth his salt will always pull the system to a vacuum and then see how well the system maintains it.