Like any piece of machinery that gets a lot of use, your air conditioning (and entire HVAC system) is going to collect dust and dirt. It's important to have your system professionally cleaned so that the parts keep working as they should. Even if you regularly change your filter (which you should!), dirt can accumulate in the evaporator and condenser coil, which makes it more difficult for the AC to absorb heat, making the whole system run inefficiently. The fins and drains also need to be cleaned to increase efficiency and reduce the workload for your AC unit.
While your system is being cleaned, the technician will also inspect all the parts to make sure they are in working order and that there is no damage. This inspection is important because if any problems are detected they can be addressed right away before they become bigger problems and cause major damage. This inspection checks how the blades are rotating, electrical fittings, how the motor is working, drain lines, how much electricity the compressors and motors are using, and much more.
Along with the inspection, your HVAC system will have every part inspected. The fan motor and blades will be checked to verify adequate lubricant levels, and in fact, all moving parts will be checked for lubrication. It's also important for the electrical components like the thermostat to be checked.
The technician will also check the refrigerant level in your system, which can alert him or her of a leak. The pressure is also checked because optimum pressure allows your system to run at its highest level of efficiency.