Rosa feels that Liesel writing letters to her mother is simply a waste of time. This is mainly because Liesel's mother is missing and they don't know what happened to her.
Rosa and Liesel are characters of "The Book Thief". Rosa is Liesel's foster mother. Liesel is the main character of the story and she is the book thief. The narrator of the story is Death. The setting is around the time when Germany was under the rule of Hitler.
Liesel was left to the care of Rosa and Hanz because Liesel's mother is sickly and can no longer afford to take care of her especially after the death of Liesel's father and brother.