- you may have an easy revenue opportunity with a downloadable app

- you may reach new customers through the app store mechanisms

- you may be able to provide a useful offline experience

As a user:

- for a daily use app, it may be more convenient to launch from the phone app mechanisms instead of a browser favorites list

In general:

- providing more routes to access your application is good. Let users use the one they like best.

Yes, I'm aware of the iOS function to save an app bookmark to the home screen. I've never met a non-developer iOS user who has used it.