Vehicle Manufacturers recommend having your Air Conditioning system serviced (an Air-Con Regas) every 2 or 3 years because the system can lose between roughly 5% and 15% of its refrigerant gas every year through the hoses and connections in the system.
A lower gas pressure reduces the ability to pump the lubricating oil around the system, leading to potential damage of expensive components such as the compressor, condenser and evaporator and the dying out of the system's hoses.
As such, as well as keeping you cool in summer and demisted in winter, an air-con regas is part of a preventative maintenance routine for all vehicles.