The General Rule
The general advice is that you should have your garden machinery serviced around once a year, but there are instances (particularly when your machines are doing a lot of work and/or use advanced technology eg robots) that you might need additional tune ups throughout the year..
Why service your garden machinery?
Think about your garden machinery in the same way you would think about your car. Would you leave your car until something went badly wrong to have it looked at? Most people will take their car to a garage as soon as something seems wrong to prevent a major expense later.
Don’t wait for a small problem to become a big problem later
When it comes to garden machinery and say lawnmowers, more people will wait until their machines have completely failed rather than fixing a minor problem when it first occurs. In this instance it might be because the lawnmower is used less frequently, and perhaps don’t necessary notice when something feels different.
Regular lawnmower servicing will pick up on any minor issues, as well as keep your warrantee valid (T&Cs apply), which means you’re more likely to seek professional help when something seems a bit off.