It's to prevent people bypassing this approval and distributing their app through other methods.
If you're just using the free developer program (by signing into Xcode with your Apple ID), this is all automated and you don't have to get into the nitty-gritty of certificate and profile management (but it's all still accessible if you want to).
There's some big restrictions on the free program though, not least being unable to use certain capabilities in your apps (e.g. multicast mentioned in the OP, but also background modes, push notifications, etc.) Most of these are self-service and don't require asking Apple nicely, but you don't have access to the developer portal so cannot add them to your app.