In Tom Wingfield's opening speech, he identifies the different characters that will play a role in the dynamics that are about to unfold. He mentions himself, his mother Amanda, his sister Laura, and "a gentleman caller." The only other character that is included is his father, or the memory of him, as he abandoned the family a long time ago.
Tom places emphasis on Jim, the gentleman caller, because Tom feels that he is
the most realistic character in the play, being an emissary from a world of reality that we were somehow set apart from.
Just as a quick review, let's remember that the Wingfields are crippled in many ways. Laura's physical impediment makes her socially anxious, feeling extremely shy to the point of illness. Her overbearing mother, Amanda, feeds the weaknesses of her daughter with pity and overcompensation, while also forcing her to her attempt independence the only way she knows how: badly. Tom, crippled by the fear of taking the first step toward his own happiness, remains cloistered in a home where the vicious cycle of enabling and escapism is consistent.
Back to the gentleman caller, according to Tom, Jim O'Connor is a "symbol," a universal symbol born out of the Jungian concept of a collective unconscious that unites all human beings at a metacognitive and, perhaps, even at a cosmic level. In this collective unconscious all humans experience sameness. Our needs and wants may not match but we all have them, nevertheless. This is why Tom says,
[Jim] is the long-delayed but always expected something that we live for.
As humans, our cognitive ability allows us to dream, hope, and want. In the case of the Wingfield women, Jim is both the "want" and the "something" that both Amanda and Laura aim for.
...the idea of getting a gentlemen caller for Laura began to play a more important part in mother’s calculations. It became an obsession. Like some archetype of the universal unconscious, the image of the gentleman caller haunted our small apartment.
Jim represents the quenching of all of Amanda's fears. In her mind, he would bring Laura out of her shell, marry her, and take care of her. This is all a fantasy; she has created an image of Jim and has taken it to another level. Jim is her last big hope.
Laura also sees Jim as an ideal man. This idealization makes him a bigger deal than he really is. Her view of Jim is a suspended animation that begins back in her high school years and has remained alive in her mind even five years after high school. No time nor circumstance has mitigated her view of Jim; he is still (in her mind) the charming teenager who dazzled the audiences with his school performances and good looks.
Together, mother and daughter have made Jim their goal, for different reasons. As such, they bring their hopes up (at least Amanda does, more so than Laura) only to come crashing down to reality again. This is the way that Jim's character relates to the universal unconscious that Tom talks about in his introduction. He is the symbol that fits the women's very human tendency to hope, wish, and dream of something.
While their view of Jim is erroneous, he was, for a while, a fantasy with which they could escape their harsh reality. It is just what humans do sometimes; find an escape, and hope for something better when reality is dire.