I'm happy to explain how I'm using it currently. The power of Scriv for me is that it can give overviews at ever increasing levels...
My dad and I are writing a series of kids' books. The first book's first draft we did entirely in OO/whatever he used on Mac. As such it was written in a linear fashion and the problems we found were when it came to redrafting. Even a few thousand words is difficult to keep tabs on what is happening where when two people write it, especially in shorter scenes used for kids' stuff.
The second book was started straight in Scriv once we had talked and had a very, very basic idea of what we wanted it to be about. The image below is an idea of how we're going about this...
"Book overview" down in the bottom left is the book separated into the theme of each act, along with ideas of what we wanted the plot and pinch points to be, as well as the turning point. This was our map and from that we created synoposes on the set chapters in which those pivotal points took place.
Whoever is taking charge of writing at the time will carry on from where we left of, taking the story forward, deciding what scenes are worth writing and doing a first draft of them. If we have ideas of what we'd like to see in the future we create a rough scene in the "future scenes" folder, and in our handover notes we'll mention in a bit more detail what we wanted to have happen.
We found this way of working, being able to readily see how much wordage a section was carrying, what the ideas were, the research (placed in the research folder, primarily visual ideas there as we use handover notes for specifics) and the synoposes; all this was vastly superior to having a single document. It was so useful I transcribed the first book into Scriv and we're using it for redrafting (you can compile the document just as synoposes, which helps massively for an overview AND for writing synoposes with enough info for you to know what's going on).
What scriv gives us is a way of keeping tabs of the story as a whole, communicating visually and verbally our ideas for advancing a story.
We use document notes on each scene and chapter to specify things that we'll look at on the next draft that aren't necessarily relevant to the current draft.