Bletchleyite
Veteran Member
They aren't waiting for the unit to arrive. They display it when the last service leaves, which is pretty sensible given how people act these days. If you advertised a train at a terminus on a platform before the train before left, I can guarantee you'd end up with a significant amount of people on that train before.
Given that most travel on WMT likely isn't on Advances because the walk-up fares are such good value (and don't exist at all for some journeys), that doesn't really matter. If people delay themselves by getting on a later train, tough, they get there half an hour later. If they get the train before, fine, they get there earlier!
Platform suppression on local services is not even a thing outside London* and it doesn't need to be on local services anywhere aside from where splitting and joining is taking place.
* Crikey, at Manchester Piccadilly there are sometimes 2 or even in disruption 3 London services advertised at once and people cope.