Whenever you are asked a question like this it is vitally important to begin by looking at the quote that is referred to in context and then to try and relate it to the theme or message of the story. This quote that you have identified appears when the family have a party to celebrate Lindsey and Samuel's engagement. As the family engage in many toasts and also eat brownies, it is only Buckley who aactually sees Susie:
He saw my shape and face, which had not changed--the hair still parted down the middle, the chest still flat and hips undeveloped--and wanted to call out my name. It was only a moment, and then I was gone.
Note the implications of this quote. Firstly, Susie is still "with" her family and very much involved and caught up in their lives. Although she is dead, she has not left them and is still "living" in a sense in the real world. She has not yet been able to move on and come to terms with her own death and absence from her family. However, at the same time, this ability to be with her family is profoundly limited. Buckley is only able to see her for a "moment" before she disappears again. The significance could refer to the family's own process of coping with Susie's disappearance and suspected death. Having been a very real presence in the days after her murder, Susie now only flashes up as a character occasionally. They are beginning to move on.