Photos and images are the essence of social networking. Facebook Photo Viewer aims to make users easier to view images. With the current Photo Viewer, users can view and browse more photos and even whole albums without having to go to the new page.
If you click on the photo on Facebook, a light box will open in the middle of your screen. So you do not have to go back and forth between pages or reload the page. Just close the photo viewer when you are finished.
I think this change is pretty good, though I sometimes find the upload feature does not work properly. That sort of software bug or something is wrong, I do not know.