As a locksmith, holding open a door while working on it (or preventing it from relocking) is a daily event at least, for me. This little bar will jam neatly under most, holding it solid. (In one direction at least. I also carry a 160mm version which will jam it in the other direction too, if needed, or, of course, a second door.)
It turns out it has many other uses, many of them things I’d either not have bothered with or would have (ab)used something else to do the same task. Now I miss it whenever I misplace it.