B1 · مقدمة · The author's introduction: why this book exists (v1/9–11)

In short

Sarakhsī wrote this book to set out the principles behind his commentary on the books of Muḥammad, Abū Ḥanīfa's student [B1.01]. He starts with command and prohibition because most of what God tests people with lies in them [B1.01].

Knowing the principles helps a student see what is true in the detailed rulings, and where other explanations of them went wrong [B1.01].

In plain words: Why did Sarakhsī write a book of principles? He had already dictated a commentary on the books of Muḥammad b. al-Ḥasan. He then set out the principles he built that commentary on. Knowing them helps a student see what is true in the detailed rulings, and where other explanations of them went wrong. He says complete fiqh needs three things: knowing the rulings, mastering them through the texts and their meanings, and acting on them. Knowledge without mastery makes a mere transmitter; mastery without practice makes a jurist "in one respect only". The complete jurist is the one the Prophet meant, and Sarakhsī names Abū Ḥanīfa, Abū Yūsuf and Muḥammad. He dictated the book "in a corner of the fortress of Uzjand", and he begins with command and prohibition "because most of the test lies in them".
In his words: "The principles are countable; new cases are endless": «فالأصول مَعْدُودَة والحوادث ممدودة» (v1/10)
Deeper: v1 B1.01