She should speak to some Oxford or Cambridge studengts, from her own school or area if possible. I was first in my family to go to university and went to Oxford. It is a one-off experience. For the right person it is incredible, for the wrong person it must be a nightmare as it is so full-on.
A lot of the myths are untrue, but some of them aren't and it is worth speaking to other people who have experienced it for her to find out what it is really like and whether it would suit her as a learning style (as that is one thing that is quite different from a lot of other unis)
Agree with Georgimama that colleges are also quite different from each other. I had no idea, and applied to the only one that said in its prospectus that it welcomed people doing my degree choice (which was a bit unusual). Think if I had known more I might have chosen a different college as, despite the friendships and experiences etc etc, not sure it was the best fit for my personality.
As I said, I had no idea and neither did anyone in my family or friends, but it is definitely worth chatting to people to find out.
In response to your actual question of why should she consider it, if she learns in a self-supporting way and is quite questioning and passionate for her subject, it really is a great place to study. If she likes lots of supported study, lectures and classes and feedback etc etc then there may be places that would suit her better (although I guess that Oxbridge sciencey types do get more structure than arts people?).
In life, I have no idea if it has helped me to get jobs etc etc but, then again, I resent the implication that university should be all about future employment. I know it is out of fashion, but I think developing the tools to think critically and to learn are invaluable as an end in themselves and don't think you should ever pick a university just on the bases of imagined future job prospects, IYSWIM?
Good luck to her with her choices!