Specifically, if you are looking for more info on someone from roughly the 1840s to recent times, this would be one way to see who else is out there AND is close enough to your family that they have the photos you don't, and possibly other useful things like old family Bibles or letters home from some war or another.
For example, someone wrote a book about the town where my paternal ancestors lived and there was great-great-granddad's photo, right on the front page! The only reason anyone knew it was him was that they happened to look through the book and ran across his name. None of us knew what he looked like. The photo credit stated who owned this photo and I had never heard of them. I contacted the author to help put me in touch but never heard back. If this were my own photo I could post it on Dead Fred and ask if anyone else has info on this person. Once you find one such person they might be able to identify some of the mystery people in your collection - you know, those pictures that no one ever labeled and now everyone who COULD have labeled them is pushing up daisies. You can use Dead Fred for free or take the membership option, which has a few more features, for under $20.