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..