Servicing is all about preventative maintenance and the same applies to the car air conditioning system. When an air conditioning system is operating corrosive acids are produced. The receiver dryer is part of the system and is designed to collect these acids, along with moisture. It's a bit like a filter that is capable of removing moisture from the atmosphere.
As the build up of contaminates in the receiver dryer increases, its ability to do its job decreases and eventually the corrosive acids in the air conditioning system damage other component parts. Because this corrosion is occurring on the internal surfaces of these parts it is impossible to accurately determine what parts of the system will fail next.
That's why you need to service your air conditioning system every two years, before this uncontrolled corrosion takes place and damages the condenser, compressor, evaporator and other parts of the system.
HANDY TIP: Turn your vehicle's air conditioner on for a few minutes every week or so, even in winter. This will lubricate the seals in the compressor and prolong its operational life.