I hate facebook's photo-sharing. There are only two things it does reasonably OK-- resize large photos and caching the photos you're likely to view next. The new interface overlay with the black background is really annoying, I don't know anyone who likes it and quite a few who hate it passionately.

I'm not a huge fan of the overlay, but it's obvious that the advantage to it is not during the time that you are actually looking at the photos, the advantage is when you finish looking at the photos you are back on the page that you started on.

I suspect they noticed a lot of people look at a profile, say "oh look an album", start clicking through and then decide they were done and go back to their feed. This design makes it more likely you will return to where you left off (without having to push back a million times) to finish reading the profile that you were already looking at.

I wonder if they noticed how angry people get when don't figure out that to go back to the album they have to click on the album name as it appears way in the bottom left corner in the area that looks bolted on like it doesn't belong. I wonder if they noticed how annoying it is to not be able to visit the album selection page from viewing a photograph. I wonder if they noticed how many people have to scroll down in order to see the comment thread. I wonder how many people have trouble finding or using the "like" and "comment" buttons given that the way they appear here is different from every single other instance in the entire application.

Yes, the advantage you point out is valid. It just comes with a whole pile of downsides and I can't believe they would do that for that one minor use case. I was with a friend the other day who was getting so irritated that when I said "just hit f5" to restore the old interface she was so happy she made me dinner in gratitude.

The only other advantage I can think of is that the previous-photo button is more mobile-friendly.