Scheduled visits are obligations with dates attached. A service that slips past its interval breaches the contract and potentially the regulations, so the plan has to be built from those dates backwards rather than around whatever else is happening.
Emergencies always arrive on a full day. Reserving engineer capacity for callouts is what keeps planned maintenance from being cannibalised, and a company that plans every hour ends up doing neither well.
Old machines wait for parts. Something obsolete can take weeks to source, and knowing which equipment in the portfolio is at that age lets a company stock ahead instead of apologising for a fortnight.
Some contracts lose money quietly. A building with frequent callouts consumes far more than its fee, and only per-contract figures — visits, callouts, parts and travel — reveal which renewals should be repriced.