It's now clearer why Google made it possible to use an iPhone as a security key -- the company is simplifying sign-ups for its Advanced Protection Program. As of today, anyone with a reasonably modern Android phone (running 7.0 Nougat or later) or iPhone (iOS 10 or later) can enroll in Advanced Protection using just their handset as the security key. You can get airtight security for your Google account without having to carry around a dedicated key fob just to sign in. iOS users will need to download Google's Smart Lock app, but that's the only major hassle.
As before, the sign-in requires a Bluetooth connection between the 'key' and sign-in device in order to thwart remote account hijacks. You can register a backup key as a safeguard in case you lose your phone.
Advanced Protection was primarily designed for political figures, celebrities, journalists and others who are more likely to be targets of hacking attempts. Google is rolling this out with those users in mind, especially with the 2020 US presidential election on the horizon. However, the company makes clear that this is available to anyone. If you're concerned enough about your account that ordinary two-factor authentication isn't enough, you're encouraged to sign up.