Some people sign up for credit cards in the name of family members (who are willing participants) — they want the bonus miles, but spouse or sibling doesn’t want to deal with meeting minimum spend and doesn’t themselves value the miles.
Things get tricky when it comes time to cancelling, a male voice may have a hard time calling to cancel a woman’s card (so cancel by mail or online). And meeting minimum spend either needs to be done with an authorized user card, or online.