Why integrate with native passbooks made by phone vendors?

Suppose you're building your own app or platform, whether on the web or native app for the app store. People will be adding it to their home screen or bookmarking it one way or the other.

Apple and Google have their own Passbooks / Wallets to store, fetch and redeem deals. They have websites touting why you'd want to do this as a brand.

But as someone who can develop web (and cordova) based apps, why would we want to use those APIs? What advantages do we really get from that? If we have already implemented the Web PaymentRequest API (and Safari's JS version of that) so people can make payments straight through the site, why use the phone vendors' wallet apps?

Web apps can already:

• Tell the browser to cache offline resources
• Process payments
• Generate QR codes and lots of other cool stuff
• Use pretty much any business logic, cryptography, signing, for group deals and other cool things

Maybe the ability to update cached resources via some sort of notification is something that a web based app can't do? So I suppose if you don't have internet access while presenting your pass, and it changed in the meantime, then it could be outdated on your phone. But the person scanning your pass would have the latest info.

Maybe NFC is a reason, because it's not supported by the web yet?