@InMyRoom
Why have an offer holders day then? If nothing to see?
I think they're just a "slimmed down" version of a proper open day, maybe for the minority who missed the open days or who went but didn't attend the subject talks etc.
They're always going to be smaller low key as most Unis have different departments having their offer holder days on different days spread over many weeks, so are organised on a "per department" basis meaning much more limited resources, staffing, etc.
I must admit, we got nothing at all out of the 3 offer holder days we went to. We'd already been to the normal open days, toured the campuses, went to a couple of talks, looked at accommodation etc.
Son wanted to go to Leeds again, because "on paper" the Uni looked exactly right for him, i.e. the exact degree he wanted with lots of module choices, but he didn't like the place at the open day and thought a second visit may persuade him he'd like it - in the event, the second visit firmed his decision he didn't like it and made the decision easy not to go there.
For the York and Lancaster offer holder days, they were, both, a waste of time really. The "talks" contained no new information he'd not already heard from the proper open day or that he'd already read on their websites, and that was it really other than a pretty tedious tour of the Maths building at each, showing a succession of lecture theatres and teaching rooms and a few "break out" areas, which were all pretty generic. Son only really wanted to go to make sure he wasn't missing out on anything - at both, we left early at the mid afternoon break because he'd seen/heard nothing new and the remaining schedule was much of the same. I suppose things would be more interesting if you were doing, say, a science degree, and having a tour of their labs/equipment, etc.
We knew in advance that they had separate days for accommodation tours and that these wouldn't be available on offer holder days.
So, back to your point, offer holder days are more maybe for people who missed the proper open days, or maybe who need a personal 1-2-1 discussion with departmental staff. Most Unis seem to have specific accommodation open days closer to the application deadlines where there are accommodation tours (mostly run by student ambassadors rather than staff).
We went to an Open day at Warwick and son didn't really like it and did pick up on it not seeming to have many amenities compared with collegiate Unis which had their own common rooms, bars, etc., and that it just seemed to be a supermarket, cafe and student union in the central area, and not much else!
He ended up at Lancaster partly because of the course, but also because of all the campus amenities, all the different college bars, takeaways, restaurants, shops, etc. He's not a particularly social person, and when he was living on campus, he rarely left it, as it had all he needed (particularly the Greggs and Subway!). He did say he could tell when there was any kind of open day on at a weekend as there were people about, but when there wasn't, the place was mostly deserted! He gauged it on whether there was a queue at Greggs or not!