Roxane tells Christian that she has made the dangerous journey to come to him because of the letters that he has written to her. She says that she began to know his mind and soul the night when he spoke to her under her balcony. And the letters were so powerful and so sincere, that she now wants to ask his pardon for loving him only for his physical beauty. She feels now that that was an insult, for his mind and his spirit are so much more beautiful. In reading his letters she has learned to love him for better reasons, more deeply than before. His physical appearance now means nothing to her.
This is the reason that Roxane has to make an appearance on the battlefield. Without this scene the play would be meaningless. Christian must learn that it is Cyrano whom Roxane actually loves. We also now discover that Roxane's character has begun to undergo a very definite change. She is capable of more maturity than Cyrano gave her credit for. His persuasive powers are greater than he knew, for he did not dare trust his ability to woo her for himself. And now she is married to Christian.