Williamson might not be the flashiest role in the play, but he serves a very important purpose: he's the antagonist of the story.
In some ways, Williamson is there to represent the corporate office, middle management, and book learning (as opposed to street smarts). Even his name is pretty generic (no offense to any John Williamsons out there), and it definitely lacks the specificity of a Roma, Levene, Moss, Aaronow, or Lingk. John Williamson is not a name that's going to stand out on a list of names, and this John Williamson is not a man who is going to stand out in a crowd.
This guy is pretty much reviled by all of the men in the office because they see him as someone who doesn't go out and get his hands dirty. He doesn't go out and do the work, and instead is the guy who sits behind the desk while everyone else is out there putting it on the line.
Mamet makes it pretty easy to dislike Williamson. If you side with Levene and Roma (and the others), you might take them at their word when it comes to Williamson, and they are not kind with their words toward this dude:
ROMA: Where did you learn your trade? […]You idiot. Whoever told you you could work with men? (2.1.955-957)
This little excerpt isn't even as bad as it gets. Nope—it gets a whole lot worse, and Roma uses some words that are not very nice to repeat in front of anyone… ever.
Just Doing His Job
It might be easy to jump on the side of Roma and the like and disparage Williamson for being somehow lesser than the salesmen in the group, but Mamet doesn't judge, and in the end, he actually gives Williamson the final victory. Not only that, it comes pretty easily for Williamson:
WILLIAMSON: You want to talk to me, you want to talk to someone else… because this is my job. This is my job on the line, and you are going to talk to me. (2.1.1040-1042)
Williamson is no less worried about his work than the others—he has a job and he's going to do it. On top of that, he's not going to show any respect to a man (or men) who have never shown any respect to him. Williamson not only gets Levene to confess, he gets Levene to sell out Moss, and it takes an easy lie and all of a few seconds.
Here we have the guy—Williamson—who has been abused for not being a real man, for not having street smarts, for not knowing the score and all those other things the guys like to throw at him. However, he's the one who comes out on top. He's the one who's linked directly to the guys who made the rules, and he's the one with real power. The salesmen can talk all they want, but at the end of the day, the guy with his own office is the guy who is really running the show.Williamson Timeline