1. That's where users are. We've trained a generation of users to find apps in app stores.
2. Some mobile platforms (read: iOS) are hostile to web apps. For example, Apple doesn't let you play audio in the background on iOS. (Or rather, they suspend JS execution if the web app is in the background, or if the screen is off. Thus, you can play exactly a single MP3, then my media player app will stop playing music unless it's still in the fore, which is a show-stopper.) This is just one of dozens of issues with web audio in iOS Safari; I've documented more here[0].
Native apps have no such restriction. Thus, packaging my PWA as an app got around all these restrictions. Plus it lets me integrate with the OS, for example, by showing the currently-playing song name, artist, album, art, etc. on the lock screen.
[0]: http://debuggerdotbreak.judahgabriel.com/2016/12/13/its-almo...