The team states on reddit:

By cursor movements we don’t mean mouse movement, but only the typing indicator. Mouse movement is not recorded. It is only shown when you choose to collaborate with someone by sharing access to view or edit the document to make the collaboration possible. Moreover, the contents of your documents including these movements, comments, and replies are end-to-end encrypted, so that no one, not even Proton, can see the contents of your documents.

Well clicks and keystrokes are kind of important when you’re working with a text editor with a graphical user interface. These actions have to be sent somewhere.

its collaborative. the data needs to be sent somewhere, its going to be recorded somewhere

ie, you get to see your partners activity

That and an API (both things that there are search results implying they’re on the roadmap at some point) and my use would go way up I think.