A purpose is a statement that describes the path you have chosen to take to reach a specific goal which may be a sub goal of a more major goal
I am PhotoReading this book to learn about.... so that I can
This book not another book but this particular book I have chosen to teach me about this subject [purpose] .... so that I can [goal or sub goal]
If you know what you specifically want from a book and know exactly where it is in the book then yep you go straight for it. However most people don't know that a book had that particular recipe until they've done some exploring of the book and most books are easier to explore by PhotoReading first.
Alex
