The short list worth doing on a schedule
Most plumbing failures are not random. Water heaters give warning before they let go, sump pumps fail from sitting still rather than from running, shut-off valves seize because nobody ever turns them, and hose bibs split in January because they were left charged in November. Those are the failures a schedule genuinely prevents.
For a house, an annual walk-through covering the items below is usually the whole of it. Commercial and multi-unit properties carry more, mostly because there is more equipment and more paperwork attached to it. Anything beyond this list should have a reason behind it that somebody can explain to you in plain terms.
- Water heater: check the temperature and pressure relief valve, inspect connections and venting, flush where the tank's condition supports it, and check the anode on tanks worth extending
- Sump pump: test the float and check valve before the spring melt rather than after it
- Shut-off valves: exercise the main and the fixture stops so they still turn when they are needed
- Hose bibs: shut down and drain before freeze-up, hose disconnected
- Backflow devices: certified annual test with the report filed where the municipality requires it
- Grease interceptors: service at an interval matched to real volume, with a log kept
- Floor drains and trap primers: confirm the traps are still holding water
- Sewer laterals: a camera baseline on older clay or cast iron, then a re-check on a multi-year interval