This is because the config is not the only place do derive the domain from, but it might just be wrong in telling you to set it to nil. It likely should just skip the :host key completely.
The conn does hold the domain the app is accessed by, which is then used by the helper to build up the full url. This is especially useful for places like gigalixir, where it’s likely that the app is accessable via multiple domains. What this will prevent however is using MyApp.Endpoint with those helpers to create full urls.
There is a place, which needs an explicit host: nil, which is for force_ssl:
