It's no secret that the Pecks are different from most of the people in the town of Learning. The two main distinctions? Their religion and the simple, modest way that they live. Rob knows it, too. He asks Papa more than once why they have to be "Plain People," and Papa replies simply, "Because we are" (12.86). Don't you love it when parents give you great answers like that?
We can't stop comparing the sacrifice and deprivation that Rob has grown up with and the more materially comfortable lives of the other children at Rob's school. In fact, when we first meet Rob, he's fuming after being made fun of by a boy at school because of his clothing. We also see Rob wondering what it would be like to be able to afford a bike and speaking longingly of the wonders of a store-bought coat. The difference between Rob's fam and the rest of the town highlights what's special about the Pecks—and that's what foils are for, Shmoopers.