First, it's not why. It's that this is possible. Please don't say "this is impossible because this doesn't make sense". Second, I agree with you. Normally people don't need to do this unless they hate their hard drives. :-) But there's some special cases, for example when the disk is "volatile", or when you want a huge chunk of continuous physical address and you want to tell the OS to stay away and don't touch any pages inside.

PS: An interesting thing is that Windows people always seek for a way to enlarge disk cache. It's fine on server versions, but on desktop versions such as Windows 7, if I don't remember wrong, disk cache is capped at ~4GB(not sure if globally or per file). Oh well.