Purpose helps you to build the mind probing questions. Since the mind loves getting answers and hates unanswered questions you have the motivation to find the answer quickly. Without a purpose it's difficult to ask questions that build the relevant knowledge you are looking for.
Purpose is the path you've chosen to take to reach your goal. A purpose can point out that a different book would be better than the one you have in your hand. Once you're clear in your mind what your goal is and your course of action (purpose) you won't need to change your purpose 5 or 10 minutes into the book.
If you're straying you ask different questions. If you don't have a question you don't have a reason to concentrate on your reading. Then you lose your concentration and reason for reading.
Alex
