Sometimes, the publishers want promos. A magazine wants one, or a convention does, or something, and so the word comes down to me, could we have one please. And I like to be friendly. They have been given away various ways, depending on the language, but the BGG store has had them all (in English) in the long run.
Here is what I have to say about the promos so far. There may be more!
Envoy: This started in Intrigue. It fit with the decisions theme; you give your opponent a decision. Often that amounts to just "discard the best card," and having your opponent pick just keeps you honest. Sometimes it's a real decision though. The mechanic seemed fine, but what the card did was, it was a terminal action that drew you cards. There was enough of that already between the main set and Intrigue. And the new part to Envoy was interesting, but didn't like give you a new deck to build or anything. In your deck, it was like a Smithy. So I took Envoy out, to perhaps try in a later expansion.
Then one day Jay said, he needed a promo, could I have it say by tomorrow. I didn't want to steal a card from a future set, and also, I wanted something with as much playtesting as possible. That basically meant an outtake from the main set or Intrigue. Envoy was easily the best option.
Black Market: This started in Seaside. Seaside involves your next turn; Black Market reaches into the next game. It was adored by some players - a star of the set - and seemed harmless to everyone else. It left the set though, due to the combination of rules wackiness, and the cumbersome set-up. I have never had to do that set-up - I don't have separate randomizer cards, because why print 11 cards each time I change something when 10 will do. I use one card from each pile for the randomizer, putting it into the pile to play and returning one to the randomizer after the game. So Black Market had no set-up at all for me - I already had a pile of unused cards with the proper back right there. But we included a separate randomizer deck in the main set. So there was this set-up. So, wackiness, set-up, it left the set.
Then we needed a promo, Dale said why not Black Market, and I was eager to have it exist after all. Ultimately it's not a great choice for a promo. It still has the set-up and still has the wackiness, only now you don't have a rulebook telling you how it works, you have a slip of paper you lose and then the internet. I'm still glad it exists though, it's a fun card.
Stash: This is a Seaside outtake, though it started in a later small set that dissolved, from when I rearranged everything into 16-card expansions. It seemed like a good fit for Seaside's next-turn theme. There was a victory card with a similar mechanic, which did not seem worth preserving. Obv. you put that one on the bottom.
Stash had two issues. First, I didn't have the different-back mechanic yet. So like you set them aside, or dug through your discard pile for them. It was not going to be pretty rules-wise, since by default you can't look through your discard pile. And second, it was not that interesting. There are some cute interactions, but on the whole, it didn't seem necessarily worth a slot in the set. So it left.
Later I realized I could have a different card back, solving the rules issues. I immediately put it on a list of potential promos, with an eye towards putting it in an expansion if it wasn't needed as a promo. It was though, and there it is. To me this is the ideal promo: exotic-looking, but not actually complex; interesting when you read about it, but not something I'm sad didn't make an expansion.
Walled Village: This is an outtake from the 7th expansion. I replaced it with an on-theme card. A few people were sad to see it go, but all of the Village slots in the other sets were full, so it was dead.
Then Jay wanted a Carcassonne-related promo card. First I looked at designing a card specifically to fit with Carcassonne, and well Carcassonne does not have a lot of ground in common with Dominion. Then I remembered this card, and it seemed plausible to call it Walled Village. Jay went for it and there it is. These stories can't all be interesting.