In general, it's a matter of rapid prototyping for POCs or MVPs that don't need much. As an example, I'm currently working on a system that needs a minimal iPhone/Android app. Just a login view and a couple buttons. I'm the only engineer working on this, and I have way more important stuff to do than manage two completely different codebases right now.
So for my MVP, I'm planning to use React Native to build the minimal interface, and then (once funding clears and we can figure out the hiring situation) I'll get one or two engineers to work on building a native app for each system.
iOS and Android projects look nothing alike. Sure, on the grand scheme of things, comparing the complexity of the backend to an application that is basically two views, the application with two views is a tiny percentage of the whole thing. But duplicating that percentage right now doesn't buy me anything.
So for my MVP, I'm planning to use React Native to build the minimal interface, and then (once funding clears and we can figure out the hiring situation) I'll get one or two engineers to work on building a native app for each system.