C-x z (repeat) is a little bit of a mystery to me.
That is, I don't know when you would use it.
I mean, if you need to kill five lines, giving a numerical prefix to the
kill line command, like M-5 C-k, makes good sense to me—you have an
idea of how much you need to delete, do it all in one fell swoop.
Or if you've got a simple set of commands you need repeated, whip up a
small keyboard macro, and then repeat it with
C-x e (kmacro-end-and-call-macro), and then keep hitting e until
you're done.
But I've been trying to figure out when I'd need to use C-x z and I
can't come up with a situation where it's not going to be as many
keystrokes as whatever I was already going to do.
Still, hope springs eternal. Now that I'm aware of it, perhaps I'll start noticing uses for it.