Software clutter builds up
Programs that start automatically, temporary files and browser extensions accumulate quietly, and a PC that felt fast when new can feel sluggish within a year or two without anyone changing anything.
Hardware needs attention too
Fans clog with dust, drives fill up, and older mechanical hard drives slow down as they age. None of this shows up as an error message, it just feels like the computer is getting old.
A yearly check is usually enough
A once-a-year clean, software tidy-up and quick hardware check catches most of this before it becomes a real problem, and is far cheaper than replacing a PC that only needed a service.