There is no fixed schedule and servicing plan for cameras and lenses. We can only guess about the driving factors of these 'some photographers'. Two possible reasons could be:
- Recent exposure to very harsh conditions (dusty, wet, cold) require a cleaning and thorough check
- Prophylactic check before important assignments / trips to avoid equipment failures
For hobbyists, the simple rule is: get if fixed if it's broken. Or of the manufacturer has issued a service recall..
'Some like to send' sounds like a hobby. Generally a rule of thumb to send in for me is after a trip , especially after the camera has been exposed to harsh weather (eg: seawater splash onto my camera while I was seated on a small boat trying to shoot seagulls once)
If not if it's not broken , don really need to send it in.
Another guide some of my friends do for peace of mind is annual send in just for a clean up (sensor etc) and a checkup at the same time & these are people who use their cameras nearly on a daily basis.