Well, perhaps straight teeth are less prone to cavities? Or maybe having teeth better aligned for chewing helps with your digestion, or makes you less likely to get gum disease from food being shoved against your gums, or something? I know in the case of my palate expander, it was not only cosmetic but also so I wouldn't need teeth pulled at a later date, but I'm not sure that the palate expander was preferable over tooth-pulling.