If reading a book where you know generally the location of the material you need, what purpose should PR serve? I had been using PR as a tool to locate information in text that gave very little guidance i.e. not many subtitles and key words etc...
Photoreading helps connect the information to other material that may not be identified as such in the text but also matches your purpose. It helps you to recognise the whole of the subject rather than just the parts. Allowing the inner mind to link the other relevant information to you purpose even though you may not need it yet.