People in abusive relationships sometimes have secret phones in case theirs get taken by the abusive partner. I think a lot of the warnings I've seen are directed at those people.
I think it's more directed to victims of abuse or domestic violence, who sometimes try to keep a phone hidden from their abuser that is outside of their abuser's control.
A lot of people also have two phones due to less dire circumstances than those listed here or criminal related things - like people with work phones.
When there's an alert (usually Amber alerts) late at night having four cell phones screeching and awakening you from the depths of sleep is panic inducing. My husband and I both have personal phones and work phones and it's jarring to say the least when they all go off at once.