It allows her to feel young and unscarred again. The conversation about Mitch's size is a charming and comic moment. Finally, we hear the truth about Blanche's marriage. Depending on the version o the play you have, the homosexuality of Blanche's husband is either insinuated or said outright. Either way, Blanche has never overcome her feelings of guilt for what happened.
Loneliness plays itself out in so many ways throughout the play.
Since than, she has been a girl caring for dying relatives. Mitch, too, is lonely. Mitch makes Blanche feel safe. He is touchingly gentle and sensitive. In his company, the music fades away, and Blanche notices the difference.