Other “real-time” editors like Google Docs work by broadcasting an updated copy of the document to everyone every 15 seconds. This creates a noticeable lag that gets in the way of collaboration. You start editing something, only to find 10 seconds later that someone else deleted it.
Etherpad updates every copy of the document every half second. This 30x increase in speed changes the experience completely. Your edits hardly ever clash with other users’. So you work confidently instead of tentatively.
Why doesn’t Google Docs update every half second like Etherpad does? Because it’s really, really hard. We’re fairly experienced programmers, and to make this work we had to solve problems that, as far as we know, no one had solved before.