Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
"ALWAYS BE CLOSING"—Practical Sales Maxim
Man, that maxim is so powerful it deserves its own monologue. Oh wait—it has one. Phew.
This phrase plays such an important role in the play that when it came time for Mamet to write the screenplay for the film version, he added a character (played by Alec Baldwin) just to deliver a monologue about the maxim. "A-Always. B-Be. C-Closing," he says, and then goes on to berate all of the salesmen in the office.
While the monologue doesn't exist in the original play (though some stage productions have added it over the years), it does still make an appearance. As Shelly recounts his sale to the Nyborgs, Roma is impressed and chimes in with, "Always be closing" (2.1.432).
This is the maxim that they live by. Well, at least Roma seems to live by it. Even a trip to the Chinese restaurant is a chance to close a deal for him.
The epigraph also illustrates the unbelievable pressure on the characters in the play—as Moss says, "The pressure's just too great" (1.2.48-49). There are no breaks for these guys. They have to always be closing, and this pressure is what sends them over the edge. It's what makes Moss come up with the idea to rob the place, it's what leads Levene to carry out the robbery, it's what forces Roma to spin lies in an attempt to hold on to the Lingk sale, and it's what makes Aaronow utterly hate everything about the job.