The glass castle was so important to Jeannette, because she truly believed in it. She thought once her dad built it, that their lives would change and they would move out of their impoverished home. I think as she grew older, she realized it was just a fantasy dream and it was impossible to build a castle made out of glass. And it also has to do with her dad breaking his promise of building it.
Finally when she wants to leave her parents and go to New York, Her dad comes to her and shows her his plans of building the Glass Castle and how he was going to make her room bigger since Lori was already gone and her room was empty. She tells her dad that he will never build the Glass Castle and if he did, she was already gone. I think the dad is trying to do every thing he can to change her mind and make her stay, but it is too late. I was surprised that her Mom was okay with her decision of going to New York city.
This is important to the book, because the tittle of the book is Glass castle. When i first read the book's tittle i thought the whole book was going to be about building the Glass Castle.