Is this something only for people who want to start writing, and need to start building the habit to put in time every day? Or can it also be useful for people who already write?

The main issue that pops out to me is that people who write have projects (sometimes several, sometimes massive ones), and "writing" daily sometimes means much more editing/cutting than writing fresh text (I'm married to a novelist; her first book lost about five hundred pages between the first "completed" draft and the final result). Or -- writing may mean composing new text, but it's placed somewhere in a much larger work.

Last thought! Be very careful about having ability to export from the start, and be very sure you don't run out of time / hosting fees / whatever and let it die with anyone's work trapped in it.