Who are The Huffelmeyers from Love, Stargirl and what is their importance?

The Huffelmeyers are an old couple that Stargirl's father delivers milk to. The couple always orders the same thing: 1 quart of buttermilk and 1 quart of chocolate milk. Stargirl is particularly drawn to the couple because they trust the neighborhood enough to leave their front door open for the milkman to make his delivery.