Diverted hot water from a steamy hot boiler may miss it’s requested direction if the diverter valve is defective. When this does happen, user either get domestic hot water flow and NO heating at all or vice versa.
Could a boiler annual service really stop this from occurring? Not really unfortunately. It’s a wear and tear part and only the usability and time governs how long your diverter valve assembly last before it is due a replacement.