With no tax disc issued these days, without a reminder owners loose track of when the tax expires. The reminders are sent out to all vehicles, and even the Historic tax free ones need to be insured so the reminder also brings this to the owners attention. As MM says the DVLA need to maintain records. Having said that the SORN system is a bit of a farce now that it does not have to be renewed, many vehicles that are scrapped are still showing as SORN if not scrapped via a licenced breaker.