At my institution, there are general open days at this time of year for people to get a sense of the institution and courses in the general area of study (via course talks) and then, if they apply to us, we invite them to a dedicated day in early Spring to get a much more detailed picture of what studying a particular course will be like. Potential students should go to both.
It's important that students like the idea of going to a particular institution, will feel supported there and feel ok living in that area. The benefits of university courses are heavily dependent on what students put in themselves so it's not a good start if you don't really want to study at that institution.
At this stage, it's also about getting a feel for teaching staff. Some of the research-led institutions will advertise their whizzo research names without mentioning that, because these people are busy researching and trawling for grants, they mostly won't be teaching undergrads - that job often goes to teaching assistants who are usually people finishing their own postgrad studies, TAs may be brilliant but not many are trained to teach - undergrads should be taught by a mixture of people, and the bulk of them should be experienced teachers. Published research is available to everyone, so while it's of benefit to the institution to have lots funded research there, that doesn't especially affect undergrads, so potential students should ask who'll be teaching them and should meet them and be able to chat. If you're shown around by admissions staff alone, ask yourself why.
Potential students should ask about resources for the courses they're interested in: a crucial factor being numbers, you may get shown great kit / facilities but how many undergrads are sharing them is another question. You won't be able to tell how good a library is from looking at it, most resources are electronic and online now. How much subject-specific scholarship and other informations resources is subscribed to is an another question to ask. Beware of institutions who don't have IT facilities for all students: everything students research, write and submit at my institution is online now, IT provision (fast connections, universal wifi, enough easily-accessible computer terminals for every student who needs them them to work) is essential these days, so ask about that.
Also, if an undergrad is intending to stay in halls, see them, the full tour, and pay attention to the literature describing them and other campus facilities (watch out for split sites). Most will move out in second year, so ask about where people normally move to and how the institution supports that. At any decent open day, you should be able to talk to undergrads who are doing the course you're interested in and those who've graduated from it.
All these things matter because students . Most potential students come with parents, and we split them up for different tours and talks, so the young people have a chance to be themselves and chat on their own behalf but because they're also shy and won't necessarily think of pragmatic questions, parents can be a good back-up, asking those questions for them. As long as parents keep themselves mainly in the background and don't hog / Gestapo-interrogate the staff, overbear their own embarrassed children or try to make their children's minds up for them, they're a help usually. I'd go with them, at the Autumn open days for preference.