What do all the purchased cards (ie. Ace of Gomorrah, etc.) on the different merchants do? Playing caravan does not allow me to use these in my deck.

Ringo in Goodsprings will actually tell you in a little bit of detail about those cards. The Holotape will also explain a little about it as well.

The cards are part of sets that can be found anywhere in the world, You can add these cards to non-collection specific decks, so if you wanted to you could add it to your deck you have already, if you don't have the same face card already in your deck. I'm pretty sure the first deck you get is a complete one, so there's no adding new cards to that one.

Can I get more of a spoiler or detailed explanation? I've done Ringo's quest, but don't remember getting a holotape or explanation on this. So if my basic caracvan deck is a full deck, why do I need other cards to build other sets? Do they have some sort of added benefit? How do I add these cards to my deck or how do I get/create new decks?

I can't remember if the holotape actually came from Ringo, but it did come from someone in Goodsprings. Pretty sure it was when you first meet him, though.

As of now the decks are apparently bugged, and will not work. I don't have any other full decks besides the default, that'd take a while to get the cards for, heh. No idea on added benefits, but it would make sense seeing as how much work is involved with collecting new cards.