Note how Amanda plays the revelation scene for all its theatrical effect. This is also a part of her character and prepares us for her giddy actions when the gentleman caller comes.
This scene also prepares us for the coming of Jim, the gentleman caller, because he was a high school friend of both Tom and Laura. With it now established that he was Laura's high school idol, we are more prepared to accept her nervousness when he arrives.
Even though a few minutes earlier Amanda was able to face reality enough to discuss what happens to unmarried women, now with the thought of a gentleman caller coming, she suddenly resorts back to a world of illusion and refuses to allow Laura to refer to herself as crippled. She then tells Laura to develop charm to compensate for her slight defect. For Laura to develop charm is totally impossible, and furthermore, the type of charm that Amanda means would destroy what innocent appeal Laura now possesses.
