It's terribly inefficient for emacs vi emulation mode to actually quit emacs when you type ":q", because it takes much longer for emacs to start up than vi, and people use a long-running emacs a lot differently than they use disposable vi's.

UniPress[1] Emacs's vi emulation mode would actually flip you over to an emacs shell buffer when you typed :q, and the shell would recognize when you typed "vi foo.c" and flip back over to a vi emulator buffer instead of actually running vi, but INSTANTLY, since changing buffers in a running emacs was much faster than actually starting up a new vi process.

So die-hard vi users didn't have to re-learn their muscle memory, and could just stay in the same emacs all the time, while the same old emacs alternately flipped between pretending to be a shell, and pretending to be vi.

[1] "Evil Software Hoarder Emacs": https://news.ycombinator.com/item?id=26113192