Things break. Like chipped keytops, broken hammers, broken strings.
Most things can be fixed. some are easy fixes, some more intricate.
There are still Instruments floating around out there from the late 1800's right up to modern day pianos. Servicing 100 year old pianos is often the case for piano technicians, and with anywhere up to 10,000 mostly moving parts, something's gotta give.
Broken strings
Broken hammers
Sticking keys & pedals
Chipped ivories and plastics
Wobbly hammers
Loose tuning pins
Notes not sounding
Clicking noises
Squeeking pedals
etc.