Atticus is old and Scout and Jem want to be proud of him at school with some soft of bragging rights.
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Scout and Jem are under the impression that Atticus is old and boring. The shooting of the rabid dog, Tim Johnson, surprises them. Atticus is modest, at best, about his accomplishments. His expert marksmanship is not known to his kids before they witness their father do what most could not. This validates, in their eyes that Atticus can excel at something that "traditional" men in Maycomb might strive for.