From there, I can run the command I am interested in with a single
keystroke, and the transient menu disappears.  Overall it's pretty
unobtrusive[1], so once the command makes it to muscle memory it's
virtually indistinguishable from just running "C-c @ <key>" without
pause.

This is a pretty trivial example; see Phil's answer for more interesting
features (e.g. providing arguments to commands).


[1] Unlike say C-h b or C-h m, which 1. takes a whole window, 2. I have
    to scroll through if the key I'm interested in isn't featured in
    what's initially displayed, 3. I must quit manually.