In the age of audio streaming your customers do not want to download and mange MP3 files. MP3 files are obsolete. Customers may not want to enter credit card information into your website. With the App Store customers can purchase your content in a safe and secure manner without the risk of piracy. MP3 files are easy to copy and pirate. With apps customers can not giveaway your content to their friends. With an app your customers do not need a player, because the app is a player.
It is easier for you to promote your app anywhere at anytime, because millions of customers are accessible to Apple's App Store.