I'll share something that I discovered from my experience with photoreading and setting my purpose.
Another way of looking at your purpose is to view it as your intention. Intention is more action driven. Knowing what stocks to invest in and actually investing in the stocks are two totally different things. Can you see the difference?
When setting your purpose (intention) what do you want to do with it? Do you want to know what is the best value vitamin to buy your family? Do you want to know what vitamin will help you with a health condition?
Knowledge is trivial unless it serves an end benefit. Does knowing that Vitamin C may sometimes be called ascorbic acid do anything for you? Probably not. Knowing that Vitamin C can help curb your sugar cravings and help you lose weight or that it can speed up your recuperation after working out so that you will have more energy faster can serve your intent of better health.
Does that help?