Y? Is the most important question anyone should ask for. Y? gives you the context of How? And What? Once you know Y? What? And How? can be replicated accordingly.
Y? Brings you to the core of reality as it is. All other question like
What? How? When? Where? Whom? Are purposeless. Y? Is the purpose and context of all questions.
When you don't understand Y you are doing what you are doing? Then not only get clarity in life but also most of your problems will be sought out. The fundamental of Y? gives you seeking and acceptance that you don't know many things. People don't want to ask or answer Y? because they feel fear of uncertainty or not knowing the answer. Y? Can give you awareness. Ask Y? Till you get the answer.
So, that's why the author doesn't want to spoon-feed you with stories or information but wants you to ask Y? whenever you feel stuck or not able to understand in this story and life in general.