Informed Webmaster

If you make your site responsive so it works on portable devices why would you need the added expense of developing an app and maintaining it?

Responsive Design

Responsive design is an approach to web page creation that makes use of flexible layouts, flexible images and cascading style sheet media queries. The goal of responsive design is to build web pages that detect the visitor’s screen size and orientation and change the layout accordingly.

Definition https://whatis.techtarget.com/definition/responsive-designresponsive design at Whatis

Website App

...a web app is a website that is designed fluidly, responding to being viewed on a smartphone. There are many different types of websites out there, some are static and rarely updated, while others are responsive and have a great deal of interactivity. Web apps, specifically, function like downloadable apps, but all from the comfort of your phone’s browser...

What Is The Difference Between A Mobile App And A Web App?

From the definitions above they both basically accomplish the same thing. Give the user information on their chosen device via a browser and allow them to complete tasks on the website.

So why have both?

I can't see any reason why. The only possible reason that I can think of is possibly the app uses less BW.