Well, this is a new way to spot an American in the wild.

WhatsApp is basically cross platform iMessage. That’s why many parts of the world use it.

Always has been. Many Americans, myself included, never used WhatsApp since SMS between people we have needed to contact was "free" since like 2007.

While I do use iMessage a lot, I have typically used Facebook Messenger for cross-platform.

But now I am using RCS more and more since it's on iPhone now.

Given most messaging apps are pretty much interchangeable from a UX perspective, the main reason you would use it is that people you want to talk to are using it.

If you live in the USA, that's probably not the case. If you live in Europe, it probably is. If you live in Europe and there's a group chat taking place, it's almost certain to be in WhatsApp, and that's a much better UX than if it was SMS.

SMS is also unencrypted, fairly spoofable and offers no delivery guarantees.