For one, google docs has a free-form dynamic that doesn't really prevent people from cannibalizing each other's work. PenFM, like its predecessor Neovella, enforces a turn-based writing collaboration. But that's entirely just in terms of how it differs from GDocs in the sense of sharing the production of content.

Collaboration in PenFM happens in another, more powerful way: the ability to react to parts of a story with simple likes and more thorough commenting. Imagine having written a story, and now you have to send it out to friends & peers to review and give feedback on. Would you really rather have a back-and-forth over IM, constantly having to find the part they're referring to? On PenFM, they just click the section, type a comment, and you get notified to go take a look at what they wrote. It's asynchronous.