The Glass Menagerie is set in the early to mid 1940's. These are difficult times where the fortunes of the gilded age decades before cannot be reached. It is a time defined by its gloomy feeling, almost as if the world knows that the War is approaching. Men certainly carried the weight of these tumultuous times. After all, they were heads households and in need of money during a time in which there was not much of it.
Jim is the only character, out of all four main characters of the play, that can boast to have achieved something in his life. Although his dreams only come true in high school, Jim could easily say that he did reach his potential at some point. As a football hero, he is the most popular high school kid of his class, the most handsome, and the one voted "most likely to succeed." Life has other plans for Jim: He ends up stuck in the same low-skill job as Tom, perhaps lives with his high school glory as the only testament to what he could have become.
To Tom, Jim is a distractor from the misery of the house. He brings Jim over to his house obeying his mother's requests for a gentleman caller for Laura (and perhaps for her own ego as well). He views Jim in wonder: Was not he to be successful? What happened? Yet, Jim is not influential in Tom's life. Tom has a mind and a wish of his own.
To Laura, Jim is her Prince Charming. She is also sort of stuck back in time and still remembers him from High School . She still has not given up the crush on him. Jim is the ideal hero, to Laura- The dream that she cannot achieve.
To Amanda, Jim is her last hope. She knows for a fact that her daughter has a limp, is socially inept, has terrible anxiety issues, and is abnormally shy. Yet, Amanda's relentlessness makes Tom invite Jim over to woo and feast Jim in order for he to notice Laura. Sadly for Amanda, her efforts were in vain: Jim is happily engaged to be married. He has moved on.
Therefore Jim was the last beam of light at the end of the tunnel. He would have broken the dark and heavy spell that seems to surround the household. He is too late. Everyone in the Wingfield family is always too late. This is why Tom decides that he, too, should save himself...and he leaves them.