The candlelight gives a surreal quality to the last scene, like most of the Wingfield's lives it separates them from reality, putting them in temporary dimness, just slightly out of focus, like the characters.
The candlelight is required because the lights go out, Tom fails to pay the bill.
The dim light allows Laura and Jim to have a romantic moment, that will be brief, before the darkness descends on any possible relationship or romance between the two.
The candlelight could represent the fading light that is going out of all the Wingfield's lives, soon they will be enveloped in emotional darkness when Tom leaves the family. They are already literally in the dark, with no lights.
Amanda's world goes dark when she finds out that Jim is not available and then when she argues with Tom about it, and he subsequently leaves the family.
Tom's world certainly goes dark after he leaves his family. He is tormented by guilt for the rest of his life with regard to leaving his sister.
In Tom's final speech he says:
"The cities swept about me like dead leaves, leaves that were brightly colored but torn away from the branches. I would have stopped, but I was pursued by something. Oh Laura, Laura, I tried to leave you behind me, but I am more faithful than I intended to be!" (Williams, pg. 97)
In the last scene of the play after the lights go out Amanda instructs Jim 'the gentleman caller': "But sister (Laura) is all by her lonesome. You go keep her company in the parlor! I'll give you this lovely old candelabrum that used to be on the altar at the church of the Heavenly Rest. It was melted a little out of shape when the church burnt down." Sc.7. The symbolic importance of this situation is:
1.In the production notes to the play williams insisted that "the light upon Laura should be distinct from the others,having a peculiar pristine clarity such as light used in early religious portraits of female saints or madonnas." Williams wished that the candle light would spread a halo around Laura and foreground the semi divine quality of Laura.
2. In Sc.6 Williams describes Tom as a person who "seemed to move in a continual spotlight." In Sc.7,however, the candlelight focuses on Laura and places her in the spotlight in order to highlight the change in her character:"In playing this scene it should be stressed that while the incident is apparently unimportant, it is to Laura the climax of her life." Sc7.
3.Most importantly the candlestand itself "is a little out of shape" -like Laura who is slightly crippled-but it spreads a beautiful halo in which Laura is finally cured of her inferiority complex so that she can lighten up the lives of others