Jim Bayliss has an important role in the final act of the play, he explains to Kate that he knew all along that Joe was guilty. He also tries to reassure Kate that Chris, who has run off since learning that his father truly was guilty, will come back.
Jim reasons it out in this way:
"Oh no he'll come back. We all come back Kate. These private little revolutions always die. The compromise is always made. In a peculiar way. Frank is right--every man does have a star. The star of one's honesty. And you spend your life groping for it, but once it's out is never lights again. I don't think he went far. He probably just wanted to be alone to watch his star go out." (Miller)
What Jim tells Kate, and the reader, is that Chris had his idealism shattered when he discovered his father's deceitful behavior. Chris was incapable of believing that his father would so indifferent to the suffering that was endured in the war, so much so that he would knowingly add to it by shipping faulty cylinder heads that would cause planes to crash.
Jim tells Kate and the reader, that Joe was never really completely safe even in his own backyard from the consequences of his actions, even though he lied over and over again trying to convince himself that he was innocent.
Jim's presence is very reassuring at the end, he brings a realistic perspective to the events that just transpired, the arguments with George, the fight between Chris and Joe.
He is a reasonable, intelligent man, a doctor, so it is easy to believe in his words. He speaks like a philosopher, but his explanation is very realistic, because he had his personal revolution once too.
Jim makes the reader believe that Joe Keller can be forgiven, but believes that it would be better if Chris did not return to the family home right now. Chris needs to make his own way, away from Joe and his factory.