From what I understand, the advantage of this is that you can ignore most of the pre-requisites for subjects. E.g. engl2015 (making up random numbers here) normally requires 12 junior credit points in english but BA(Adv.) means you can skip the junior points and go straight to the good stuff. Somebody correct me if I'm wrong.
Yep, I believe the new (commenced in 2007) BA Adv essentially knocks out the first year, so you go straight into second year and complete Honours in three years. I suppose it'd be good if you're confident enough to dive straight into the deep end.