Let's take the average complex development project nowadays.
Think about the best way to figure out what it is exactly that the customer 'really' wants us to implement.
His mind, his perceptions and expectations, all those technical documents, the actual reality of the situation. That which is promoted at the end of the first release. The so-called roadmap to the final release.
You have to be a psychologist, philosopher, technologist, information analyst, paranormal guru and expert on general relativity at the same time in order to understand these things.
I'm sure glad I fit this demanding profile.