Because it answers fundamental questions about religion. In fact, 1/3rd of Quran is about religion(s). And how to think and reflect about it.
For Example:
- Mankind religion was one. Why the division happened?
- How to seek evidence of correct religion and tools of reasoning outside leap of faith. By thinking and reflecting
- Who is God? i.e. Chapter 112
- What are the attributes of God and his names? Almost in every verse.
- Why it is impossible to have many Gods - logical reasons (Chapter 23)
- Guide to find the correct God from many false gods through Abraham journey and his fascinating logical debate with his people (chapter 6).
- The reason for sending prophets?
- Who was Jesus ? Why he should (be) respect ? What happened to him? Chapter 3,Chapter 6 Chapter 16.
- Where and when the corruption in religions happened?