I am a long-time user of short-lived Emacs sessions who is now trying to
take advantage of having a browser that runs for days or weeks at a
time—I've configured my desktop shell to auto-start emacs --daemon
when I log in, and I rarely restart it (though it's probably still do it
more often than a really hard-core Emacs user would).
As a consequence, my minibuffer history starts to fill up with good stuff that I want to re-use in order to increase my efficiency.
I have little doubt that at some point in the past I've typed
C-r (isearch-backward) to do an i-search-backward on the minibuffer,
only to be rebuffed (see what I did there!). And then I probably started
hitting the UP (previous-history-line) key to get back to where I
wanted to be, assuming I didn't just give up entirely.
To know that M-r (previous-matching-history-element) and
M-s (next-matching-history-element) start forward and backward
searches through the minibuffer is pretty nice. And if they're not
isearch, at least they're regexes.