Hello!
If the paint you got on the model is flat (=porous), then the wash is going to permanently discolour it, everywhere it gets on. If you only want the discoloration be in panel lines, detail and so on, then gloss-coating the model closes up the pores of the paint and then the wash only discolors the details mentioned above.
Hope it helps, good luck with your build and have a nice day
Paweł