The old house that burned and caused Maggie to be burned and disfigured is symbolic of the older part of their life, the part that Dee wanted to escape so badly. Although she is not happy that Maggie was injured, she was happy to see the house burn down, believing that it will allow her to move up in society and be better accepted. Instead, Mrs. Johnson rebuilt, and although she enabled Dee to move past their beginnings, she knows the need to have a solid residence.
I see her standing off under the sweet gum tree she used to dig gum out of; a look of concentration on her face as she watched the last dingy gray board of the house fall in toward the red-hot brick chimney. Why don't you do a dance around the ashes? I'd wanted to ask her. She had hated the house that much.
(Walker, "Everyday Use," xroads.virginia.edu)
The burned-out house symbolizes the separation between Dee and ther family, being what she saw as the destruction of the old life, allowing the new life to come in. It also symbolizes the pain that Maggie had to endure, making her insecure and shy; Mrs. Johnson acts as a buffer between Maggie and the world, and unconsciously resents the way Dee acted towards them during their times of need. In this way, the burned-out house is a catalyst for the transformation of Dee and Maggie, helping them change into two very different people.