Why is AppDelegate.swift window an optional?

Question

I was reading Apple docs, when I found this sentence:

The AppDelegate class contains a single property: window.

va

Answer 1

You may not always need it. For example, when these two methods are called :

application(_:performFetchWithCompletionHandler:)
application(_:handleEventsForBackgroundURLSession:completionHandler:)

your app will not be shown to the user, so there is no need for a window.

As always, more in the docs

Now, I'm not sure that this is the inherent reason, but it seems as a good enough possibility (at least for me). Though if someone is able to provided some more information, I'd gladly learn some more also.