Why does nginx need an auth_http entry for smtp reverse-proxy?

I'm an apache user normally, but I need to start using nginx as a reverse proxy for smtp (postfix).

I've seen tens of examples online. All of them have an auth_http entry with some php file, which I completely don't understand. This is not the first time I use proxies, and I currently use haproxy since months. So why do I have to provide a php authentication page? Why can't I just use nginx as simple as I use haproxy, and just tell it: Set a frontend here with port, decrypt ssl with this key, and finally take it to that backend/port?

Could someone please explain how necessary that auth_http entry is? The nginx manual doesn't seem to say much about the reason why this is necessary.

Thank you for any efforts.