This is the employer working around having to purchase and maintain a phone inventory for employees.

While we're on the topic, this also applies to laptop/desktop hardware for the work-from-home crowd.

In general it's a bad idea to use personal devices for work. Companies that don't give you a choice are being cheap and disrespecting of privacy at best, and want to spy into your personal life at worst. It's also really, really, really bad IT security for everyone involved.