The way I understand it is that if there is a subscription portion to your app then you're allowed to offer it via your own website but you must also offer it for in app purchasing through the Apple system for the same price. Apple will take a 30% cut of this. You can't send your app users to your website from within the app but if they are on your website for other reasons then its perfectly okay for them to purchase a subscription there which doesn't have the 30% Apple cut.

In the case of the Netflix app, I'm not sure. I assume that you'd have to pay Apple their 30% if you do have it...

This stops app publishers from charging extra for an in app purchase via the Apple system to recover the 30% cut that Apple take. It also stops people from ignoring in app purchases and just sending people to their own website to completely avoid the Apple 30% cut.

If purchased via the Apple system then you don't have any much information on who purchased it. Apple say that they'll have an option in the future which prompts the purchaser if they want to share their information with the publisher of the app but it will still be an optional step.