The letter more or less, based on historical guilds of reality, might well be a writ of craftsmanship that denotes the individual as a master or a certain occupation, [@rush99999]. As far as roleplaying goes, might be a bit easy to say, convince a local blacksmith to help you in turn for teaching them some techniques of your shared trade - trade secrets - essentially. How would he be so convinced of this? Proof you are who you say you are and what you show them. Sort of akin to a scroll of pedigree, just on a smaller scale and presumably with the obvious benefit that if we do meet your craftsman's guild, they will probably aid us; historically so long as someone vouched for allies of a guild, usually the guild would help them, barring the obvious.

I have no doubts there are other creative uses of it, such as providing it to the governor as proof you aren't just some wandering vagabond who needs be tossed behind bars for their vagrancy, or that you can attempt to use it to get work into a place you wish to infiltrate among others.