Failures of commercial refrigeration equipment to function properly (if at all) are some of the most expensive devices you could own in respect to both energy costs & the loss of revenue from improperly cooled products. Just like an automobile refrigeration equipment consists of moving parts that will need to be properly maintained so it will work for you year after year. In order to avoid expensive repair bills, having your refrigeration equipment inspected and maintained on a regular basis will actually save you money over the lifetime. Not to mention - regular maintenance can stop, if not considerably slow down, emergency calls from ever happening. Using the car analogy - you don't wait until your car completely brakes down before you have it looked at. At least, that is, you're not supposed too :)
A properly maintained HVAC unit works by removing the heat from the area to be cooled. It does not pump in cool air so much as removing the heated air from where it's not wanted to a place you could care less about (i.e. outside). The part of the system which is responsible for this function is the condenser (the outside unit). If the condenser is dirty, it will have trouble releasing the heat it is trying to remove. The result is that it will take more energy for the condenser to do its work which in turn creates more wear and tear on the system itself.
To have your refrigeration equipment checked and maintained on an annual basis means that the condenser will be cleaned so that it runs more efficiently. There are other reasons why you should have regular maintenance performed on the a/c unit.