Why did Bud get rid of all rocks except one?

Bud carries several items within his suitcase at all times as he journeys to Grand Rapids, Michigan in hopes of finding his father. In his suitcase, Bud carries fives small rocks, which have locations and specific dates written on them.
When Bud arrives at Grand Rapids, he meets Herman Calloway, who seems too old to be his father. While Bud is riding in the car with Herman, he discovers similar rocks with locations and dates written on them in Herman's glove compartment. Herman then notices that Bud has the same type of rocks that he used to give his daughter as keepsakes and realizes that Bud is his grandson.
Herman is overcome with emotion after learning about his daughter's death, and Bud realizes that he has been carrying his mother's memories inside him all along. At the end of the novel, Bud gives the rocks back to Herman because he feels like they would mean more to him. However, Bud keeps one rock that has Flint written on it, as a token of where he grew up.