Brandy? Maybe as a solvent to release some other ingredient from whatever form they found it. For example saltpeter was found under wagonloads of dung I believe - maybe the brandy was used to purify it somehow.
Water can dissolve KNO3, and ethanol can dissolve sulfir (slightly) and tar from the charcoal, so maybe it helped create a more intimate mix? It would also have kept down sparks and reduced the chance of ignition while mixing and grinding.
Oh I like that - safety while compounding must have been a huge issue.