Do you need to service air conditioning units?
Annual maintenance is recommended by the manufacturers, and installers of the systems. The majority of the maintenance time, will be spent on cleaning the unit.  Keeping the unit clean will help keep it running more efficiently, and can increase the life of the unit.  

The technician will inspect the unit to insure it's still in good condition, and will look for parts that may be near the end of their life.  Replacing parts before they fail, can save you the headache of replacing them when they fail (which will surely be on the hottest day of the year). Replacing part before they fail, can also help prevent damage to other parts of the unit.  

Depending on which company does the service, they may actually measure the amount of cooling the system is providing. They may also check for refrigerant leaks in the system, but likely only if the refrigerant level is low. If you have any questions or concerns, they'll likely address those at this time.

While these units can run trouble free for many years without maintenance. Keeping them clean and in good repair, can save you money and reduce the chances of a failure. Of course you can do some of the work yourself, but often it's easier to pay somebody else to do it.