The big selling point for me is the organisational features. For novel writing for instance (and Scrivener has templates for other types of writing too) it is very easy to divide a project not only into chapters, but into scenes within that chapter which can be written individually and then - importantly - seen in a file card on corkboard layout that allows them to be moved around and tinkered with. For someone like me who has an idea of the key milestones of the novel I'm working on this has proven invaluable - I can put those milestones down formally (heck, even write some of them in advance) and then see very clearly and dynamically what goes where and what needs to be done.
There's a lot more to it than this, and I'm enjoying finding it all out. Does anyone else reading this use Scrivener or similar & what impressions do you have of it?