Yeah the most obvious use for a light box is for tracings. but it can be used for transfering, inking and touching up.
I've been using it to ink the recent tastefully nudes. Once I have my sketch (Those of you who have ordered a comission recently have probably seen it). I simply flip it over and slap it down on the lightbox and ink it clean on the other side. So some of you might have a wild messy sketch on the back of your pics. It saves paper.
It comes in handy when I have a bunch of separet loose leaf or scrap paper sketches that I like for one of my comic pages. I size them properly from scans and construct the page, print it out and light box it onto a clean sheet.
...Man, I gotta get working on that thing again...dont even remember what pages I had penciled... crap.
did that help?