Why is Uncle Grady an important character?

In Chapter Seven, Grandma and the children go to Uncle Grady Griswold’s house, where they notice that somebody still lives. There are chickens in the brooder and the garden has been weeded. When they enter the house, an old lady is sitting beside a cold stove and on the other side is “the oldest man on earth.” Grandma is happy to see both are still living. Grandma asks Uncle Grady if he still has his army uniform. He says it is in the bedroom and she sends Joey after it. He finds it but doesn’t think it looks right. When he asks, he finds out that Uncle Grady fought in the Mexican War not the Civil War. He can’t believe that is possible. Uncle Grady will be there for the parade. They leave and Grandma leaves the kitten at the
Griswold’s.