The question of form is largely about beauty. Wright's quest for beauty is evident in all his structures. Patterson quotes Wright as saying that "beauty will be the evidence of your joy in your work." There is plenty of evidence of Wright's joy in his work at Samara, particularly in the copper fascia.
Copper might not seem like an obvious choice for a Usonian home because of the expense, but Wright designed Samara so that the costly copper fascia could be added later on. In addition, the decorative machine-pressed units were designed in keeping with Wright's appreciation for the ability of machines to produce uniform results without costly handwork.