Pianos require at least an annual tuning and occasional cleaning and regulation. The more a piano is played, the sooner it will require routine regulation, repairs, and parts replacement. If your hammers have deep grooves from striking strings over a period of years, the hammers will need to be reshaped to restore tone. Older pianos often need re-stringing, hammer replacements, and worn felt replacement. Chipped and broken keys may also need to be replaced.
Pianos have thousands of moving parts that can go bad for various reasons. It's better to stay on top of the maintenance on a yearly basis, which can help reduce the number of problems down the road.