In chapter 6, in what way does witnessing her mother's sadness help Meg?

The Medium begins falling asleep, worn out by the work, but shows Calvin and Meg their mothers in the crystal ball before they go. Calvin’s mother is disheveled and yelling at the other O’Keefe children while Meg’s mother is writing a sad letter to Meg’s father. This angers Meg and she stamps her foot and is determined to find her father. Mrs. Whatsit tells her that she will need that anger to fuel her search.