PowerApps is also a mobile app! Historically, mobile app development has involved creating apps for each operating system they need to run on (one for iOS, one for Android, one for Windows). This essentially triples your development work, triples your support costs, and increases the development resources you need to create business apps.
With PowerApps Mobile Apps, all of the PowerApps you create run through the PowerApps App. It takes care of the differences between the operating systems and allows you to run your apps. It is essentially a container that makes mobile apps much easier to use across mobile platforms.