Who (adverb or adverb phrase) answers the question 'Why'?

In simplest terms the answer to this would be:

When are you visiting them? Tomorrow. [Adverb]

Where are you going? Home. [Adverb]

How are you doing? Fine. [Adverb]

Why are you doing this? Because he is such a badass. [Adverb clause of reason]

The last question demands an explanation and therefore an adverb phrase or clause is required. Not necessarily so with the other three questions.