Crafts are used for Craft Checks. Craft Checks arise either when some rules text specifies (such as in First Aid), or when the GM tells you to attempt one. When this happens, you roll 1d20 and, if proficient in the associated Craft, you add your Proficiency Bonus. You do not need to be proficient in a Craft to attempt a Craft Check.
Note: Trades and crafts have no levels associated with them. This differs from live action FanWar. You either have a given Craft Proficiency or you don't.
Game Master
The blacksmith says "What? I'm not a vampire! I've never even seen a vampire! I walk around in the sun all the time. It's my favorite thing to do, other than bloodsipping. Er, I mean blacksmithing! I'm a blacksmith. Blacksmithing is what I do."
Rolls 1d20 with no bonus because they're not proficient in this.
I rolled a 6.
Game Master
You can see blood dripping from the corner of his mouth, bite marks on the side of his neck, and a corpse that looks just like Flumbo sticking out of a barrel. Yeah he's totally lying.
Game Master
You see some markings on the dungeon wall. They're poorly carved and not easy to make out.
Player
Can I do a Scribe check to try to see what they say?
The carvings are incomplete and have a lot of mistakes in them. You don't think whoever wrote them was very good at Common, but you can piece together the details. It looks like it says "Flumbo was here."
Player
I note that down in my Carnivore's Almanac.
Game Master
Noted.
The levels at which characters gain Crafts and Fluencies are shown highlighted in the table below.