:cq is useful in a shell loop that compares two directory trees that invokes vim to let you see what's changed in every file that's different. I use it often:

    ((cd /tmp/t; find . -type f -print) | sort | while read f; do cmp -s {/tmp/t,/tmp/t1}/$f || vim -f -d {/tmp/t,/tmp/x1}/$f 0<&9 || break; done) 9<&0

Typing ^C to vim doesn't get you very far, so if you make a mistake causing the loop to return 1000's of files you are in for a bit of pain without :cq. The :cq triggers the break, exiting the loop.