I do think the main purpose of the early promotional videos is to kick start pre-orders, which help fund the manufacture of the album or other band activities (BBT indicated that pre-orders for Ingenious Devices would help find their upcoming tour). So you need people to be aware of an interested enough to buy it in advance. It makes sense to have a new video every several weeks if you have a longer pre-order campaign. But it is kind of annoying from a fan standpoint to get things slow dripped when you’d rather just wait for the whole album.
In the case of Big Big Train, though, I think they're niche enough that they know their dedicated fans will pre-order anyway, and probably don't need much promotion to get their sales off the ground. If I recall, Ingenious Devices had two pre-release trailers, and a live video of "Atlantic Cable" from Aylesbury (which was used as the final track of the album). Not much in the way of singles or music videos, and I think it worked.
But I get what you mean - on one hand, it's a way to keep the band's upcoming release current in the minds of fans. I'm not sure if there are studies that prove if a lot of pre-release singles make a difference in sales or not. It would be very interesting to see how that could work, but the variables are so wild these days that it might be difficult. I also get that with the long lead-time and delays of manufacturing vinyl makes it difficult to have a "surprise" release of an album without having pre-orders fill the manufacturing minimums in those factories. Record labels probably want to be sure that they'll actually sell all the vinyl they make before having them made, so throwing out pre-orders 3-6 months ahead of time gives them more wiggle room in regards to release window. So many albums since COVID had their vinyl releases delayed because of manufacturing problems, and those issues still seem to happen, between the factories being backed up in production, and issues with world-wide distribution.
TL;DR - I get why they do it, but it just sucks sometimes. As for SW and his upcoming album - if he's got pre-release singles, I hope they don't make up a third of the final album. I know he's prone to shorter albums these days, so if THC ends up being 45-50 minutes and is only 7 or 8 tracks, doing three singles feels like I've heard a good chunk of the album already. This is why, if I do listen to pre-release singles anymore, I do it once on release date, and then that's it. For a genre of music that is very focused on "the album as a whole piece of art", it's weird that these labels pluck singles out so often when these songs tend to work much better within the context of the albums they're from.
-Marc.