Introducing the story through Madame Valmonde or a third person is that a third person can describe the story much better than a person whose life is discussed in the story. As Madame Valmonde was the person who grew Désirée up and was there in her life as her parent. As Madame Valmonde would be knowing more than Désirée about her childhood and how she brought her up in her life, it was really important to make Madame Valmonde introduce the story just for detailed and true story.
(Phrase this in your own words unless you want to get a 0.)
