The full title of the book is Start with Why: How Great Leaders Inspire Everyone to Take Action. So it is obviously a leadership book. but I would debate that is so much more.
When you read the book, you start asking yourself, not just as an organization, or as a leader, but as a part of society, why it is that we do what we do. And by finding that, can we find the truth of our work, and the truth of our day to day. Why companies do what they do matters to consumers. It helps us relate to brands and companies. It is also what makes us connect with other beings, with our families, our friends, our coworkers.
If you are starting something, and you want it to be big, I would recommend you read this book.
Here I leave you with a TED talk with Simon Sinek, the author of the book.