What is the point in closed book exams at university level?

What sort of questions are they asking you?
I don't agree with closed book exams because they are never representative of any skills you will use in a career. I'm not sure if this helpful in any way but that is how I feel.

I'm going into my final year of Oxford law and my exams will almost certainly be closed book (they did open book from 2020-23 but are switching back to closed book) and I really just don't get what the purpose of this is. There's about 2000 cases and articles that I'm expected to memorise and I'm having to focus almost all of my revision on memorising and repeating rather than actually understanding/analysing. If they're concerned about AI/plagiarising, they could just let you bring a certain number of pages of physical notes into the exam with you. And someone who usually gets firsts on their assignments (which are open book) could end up failing just because they don't have 2000 cases committed to memory.

I did the Jurisprudence FHS pre-COVID when it was closed book (and we also had back to back exams Monday to Saturday, which was horrible but still better than in my tutor's time when they did 2 papers per day).
I'm not going to comment on the merits of open book vs closed book exams, but you shouldn't be focusing on memorising case law and articles. What you should be focusing on is how the case law and articles fit into your analysis and understanding of a question. Bear in mind that you only have 45 minutes to write each answer so you really don't have the time to go into too much detail - I probably wrote only 2-3 lines per case/ article in FHS. 2000 cases and articles also sounds high to me (that's an average of 220 per paper) when studying 6 topics well per paper should suffice, but you should double-check against past papers and the exam rubrics.
Writing 4 essays in 3 hours means that you will be very hard pressed for time (this is especially so if they are handwritten and/or you plan on doing more problem questions) - ideally, everything should be off the top of your head. To be honest, I don't think I would have had the time to look at my notes much even if I was allowed to bring them in - I didn't even look at the case lists much.
Barring extenuating circumstances (e.g. severe mental health issues), failing is extremely unlikely, especially if a student is generally on top of their work. If you check the pre-COVID FHS examiners reports, there are no failures for most years. From 2015 to 2019, fewer than 10 students ended up with a 2ii or below.

yeah my brain is so fried and I do have seven mental health issues soooo I'm having to focus on memorising, in my open book collections I did have time to check my notes and never got below 65 though