I don't know how it is on Android but on iOS for example, you can make phone calls only the Phone app. For example, the Yelp app gives you the business info and when you tap the phone number it switches to the Phone app. For the most part, that's fine but having an SDK like Twilio allows you to keep the phone calls and the user inside your app.

That can be pretty interesting in terms of user experience where your user stays in the same context instead of switching back-and-forth between your app and the Phone app.