The entire plot of this movie hinges on a briefcase filled with two million dollars, so we're going to go out on a limb and say it's important. There's a whole lot of desire directed at the suitcase, from Moss's first greedy snatch to Wells's desperate desire to exchange it for his life at Chigurh's hands.
In fact, you can tell a lot about the different characters based on how they react to the money. Moss wants it, but he's not afraid to toss it when the cost-benefit analysis isn't going his way. The drug dealers and businessman want it enough to kill for it. Carson Wells just flat out wants it and sees it as a bargaining chip. And Chigurh… well, Chigurh wants it, but not as much as he wants to kill people. As Wells says, "He's a peculiar man. Might even say he has principles, principles that transcend money or drugs."
"Okay, Shmoop," we hear you saying. "But what does the money symbolize?"
Have the Coen brothers taught you nothing? Sure it could symbolize greed, or desire, and we're not saying it doesn't. But think about how the Coen brothers continually refuse to provide us with easy answers: no final showdowns, no big moral messages. In the end, the movie's moral seems to be about the essential randomness and life: there's only meaning and causality if you make it. And the briefcase full of money operates on the same principle: it symbolizes what you—"you" meaning the characters in the movie—want it to. It only has meaning because we assign meaning to it.
Like we said, no easy answers.