Williams, by stating this idea in the beginning, focuses the readers mind into such a state that he will unconsciously look for what Williams knows is already there-- the conflict of reality versus illusion.
Look at the characters one by one, determining each of their realities and illusions. This will help to establish why Williams chose this as a central theme, and what the resolution of these ideas are. Because Amanda plays such a large part in the play, we will start with her. Amanda's life is much illusion. She establishes this right away when she begins to reminisce of the south and her numerous beaus (12-14). Although she might have indeed had many gentlemen callers, it is rather unlikely that she had seventeen rich callers. Even if she did, she loses touch with reality by refusing to let go of her early adulthood. She has repeated these stories so much that she finds them to be completely true. Its almost like she refused to grow up with the times, especially once things perceived to be harder. .
Although Amanda appears to often be stuck in the past, without any idea of reality, she bounces back and forth. She first says to Laura, "I know so well what becomes of unmarried women who aren't prepared to occupy a position. I've seen such pitiful cases in the South-- barely tolerated spinsters living upon the grudging patronage of sister's husband or brother's wife. . . encouraged by one in-law to visit another" (18). When discussing the future, she seems to be very clear, and very much realistic. However, next she completely falls back into illusion when talking about Laura's gentleman caller. She unrealistically reminds Laura that her "defect" can be overshadowed by simple charm. She refuses to let Laura refer to herself as crippled (19). Not only is Amanda refusing to live in the reality, she is denying Laura the opportunity to be realistic about her disability.
Laura appears to be the most important character in the play, perhaps the main character intended by Williams.