I'm new to the game and don't understand the point of the affiliation card.  If you can mix rebel alliance, jedi, and smugglers objective sets to form your overall deck, why do you need the affiliation card?

It's an additional resource. So if you are creating the Jedi/Smugglers deck, right now there are only 2 possible Smuggler resources you can generate from objectives (Questionable Contacts x 2). If you get Han you can't play him unless you have one of the seSmuggler objectives in play. That is, unless you have the Smuggler affiliation card. Then you are guaranteed to be able to play at least 1 Smuggler card (Han) every turn.

So the point is to give you more resources of an affiliation that is in the minority in your deck. Usually people make their affiliation card the affiliation with only 1-2 objectives in their deck.

It is also a way to limit certain objective set combinations. There are some OSets that are tied to the affiliation card, and often this limits some sort of combination that would otherwise take the game out of balance.