All to do with the resale value of a new plate commanding a few hundred quid more. This may apply less to modded-out vans and to campers, but the learnt behaviour is still there. As a Yorkshireman I'm all for saving/ making a few quid where I can, so I'm not sure a sneery remark is really called for.
@Monty1987, the effect is greater for those vehicles registered in Jan and Feb, as you can say, for example, "a 2022 van on a 71 plate" all you want, but the reality is people tend to see the "71" and value it as a 2021 vehicle.