In theory the „how“ part is easy. All you need to do is pick a bot framework, connect it to your communication channel of preference, design some conversation workflow, connect all of that to your existing API, and you're done.
Yeah... not if your LOB mobile app is actually a mobile app that drives need to use while driving. All of a sudden the UX complexity skyrockets. When you only think about continuous real-time location sending, photos upload, and automatic verification of them, etc... functionalities skyrocket as well.
It's still doable and functional at the end, but and hear how.
As for the why part... the short is that it's not a silver bullet, but it can decrease dev costs by a significant margin, as well as speed up dev time, and „new version“ adoption. Also, since there's a zillion apps out in the wild your users will thank you for not having to install yet another app.