Generally, people use iOS for writing drafts and editing documents, and go to the desktop version for project management and compiling (in addition to drafting and editing). There are certainly folks who work exclusively on iOS, but I believe most who work in iOS also have the desktop version, due to iOS limitations.
Here are some specific differences:
The compile function on iOS is extremely stripped down and basic. This is major.
Another is that iOS does not support keywords or custom meta-data or collections. And iOS doesn’t have the desktop version’s automatic backup capabilities.
ETA: Oh, and another major difference is that I can use the iOS version to write a story on my phone, while waiting on line in the grocery store. Can’t do that with the desktop version. 8)
HTH,
Jim