Mechanical watches require periodic servicing to remove and replace dry oils and greases. Although watches can run on for many years without servicing, to prevent wear a regular service is best practice.
During disassembly the functionality of the watch will be checked and parts inspected for any signs of problems.
Sometimes parts might be worn, corroded, or damaged, and need to be replaced. For example this centre wheel had some corrosion due to a water leak.
Once the watch movement is fully disassembled all the parts will be cleaned to remove any dust, and clean out the old oil and greases.
Then the process is reversed with reassembly and new oil, again with parts and functionally being checked along the way.