I would imagine something like this could be useful to a student, who might want to download a particular site onto their laptop, to enable them to work while on a long train journey back to campus following a visit home.
Although I rarely use the browser, doesn't Internet Explorer have something similar built-in? I'm sure I've seen a "Make Available Offline" option hidden amongst the menus. As for myself, although I don't have BB, I live just 20 yards from a large public library with about 50 PCs available for free use. Therefore, I rarely reach anywhere near my 200 hours a month free-call allowance with my home ISP, so offline browsing never enters my head.