Hmm. I guess people can correct me if I"m wrong but I think that the lacquer thinner is used for the weathering all the little nicks and scrapes. After you apply a layer of paint (like a dark grey panel) You take a thin paint brush, dip it in thinner and dab the area you want a nick and the thinner will remove the paint in that little spot, It may even create some "damage" to the surrounding area of the spot making it look a little more natural.
Thinner can also be used to thin out paints so you can use it for weathering lighter panels with dirt. Dip the paint brush in the dark paint, then take the paint brush and swirl it around in a cap full of the thinner. You end up with a very liquidy "dirty" thinner that can be used to splotch all sorts of dirty goodness onto your model or into the panel lines.
It's really tough to do correctly because it's so watery. You really need to be precise and "control" where the liquid goes. It takes A LOT of patience and a lot of trial and error.
.....I think. :angel