A service visit is not the same as a repair. The device is examined under magnification, the filter and dome or tubing are changed, the microphone ports are cleared, and the whole thing is listened to through a stethoscope so a weak or distorted receiver is heard rather than guessed at.
Then it is measured. A device can pass a listening check and still be delivering considerably less than it should, and the only way to know is to test it. If the output has drifted, that is either a fault to address or a reason to re-verify the fitting.
Most of this is quick, and most devices that come in sounding dead leave working. It is also the appointment where an annual hearing check makes sense: if your hearing has moved, the device needs reprogramming rather than repairing, and no amount of cleaning will fix that.