As I understood it, the dockapps of the Windowmaker world were basically a work-around for the fact that the X11 had no concept of a dock icon (or a dock at all, for that matter). On Nextstep, the normal Mail app would show status in its dock icon, much like Mail.app does on macOS today, the calendar app would show event reminders in its dock icon, etc. Your X11 mail app wouldn’t know that there was such a thing as a dock, so you had to run a separate little app to get the “biff” function in the dock. Then the Windowmaker afficionados took the concept and ran with it, creating lots of little apps that had nothing but a dock icon, a thing that didn’t really exist on Nextstep.