I love the split screen. I'm not saying it's my single favorite feature, but...it's definitely up there. With split screen, you can opt to split your screen vertically or horizontally and view two different documents at once, side by side. I shouldn't have to tell you how valuable this is for a first draft when you're trying to remember what you wrote a few pages earlier but you don't want to move away from your current spot, or for a second draft when you're revising work you've written earlier.
If you know me, you know I adore Scrivener. I use it for many hours every day. And since it's an incredibly rich program, I enjoy hearing about how other writers use it.