I once saw someone who had "export DE=openbox", and so I changed accordingly, it was "kde" before, even though I'm on Openbox.
But now I realize that it affects things like xdg-open, which is set by configurations that I think aim to set almost-universal file associations. I think that perhaps Openbox "doesn't even count" as a DE to the related settings, so the result is that, for example, you can't open some stuff with the associated app from google...