This is most needed when the compressor and evaporator are at significantly different elevations, or excessive line set length requiring additional refrigerant in the circuit.
The compressor high side pumps a vapor. When liquid refrigerant can migrate to the compressor or flood the evaporator it keeps this separated.
It also has to do with the type of TXV that the system uses. If it is not an equalizing TXV (commonly an older style - design), the liquid refrigerant backs up on the compressor. The change of state causes excessive pressure for the compressor to deal with.
Ken, a pump daown is used in refrigration defrost cycles like you are talking about. When a freezer defrost heater comes on, it adds too much heat-pressure to the compressor when it comes out of defrost. Also a pressure regulator can be used to control the amperage draw and overload of the compressor.
Refrigeration and air conditioning are not the same animal however.