When going through old photo albums, personally, I'm usually looking for those photos that
1) Capture a parent, grandparent, or great grandparent at an age I didn't know them. Seeing pictures of them in their youth, especially on those rare occasions when they are actually goofing off in front of them camera, are priceless. There are rolls and rolls of my siblings and I making faces at the camera, but film cost more back then, and most pictures were, alas, posed. The portrait shots are wonderful to have, but I know my grandfather at age 14 didn't wear a suit every day. He probably felt as comfortable in it as I did at 14. Luckily both of my grandmothers when they were 18-19 had their own cameras, they took some wonderful candid shots, and they preserved the photos in albums. (The photos are hard to extract from the albums, but they're there, and can be scanned.)
2) Capture an ancestor or relative I never knew at all. Here candid means less, as I'm trying to get a picture of them in my head. Trying to detect family resemblances between them and those I do know.
Every once in a while I'll come across a photograph where the background is actually more interesting than my ancestor. Take this photo of my grandfather, for example. His face is in too much shadow to really appreciate it as a photograph of him. I have a few other photographs of him from his service during WWII that are much better. However, what's going on with the Sphinx?
On the back of the photo is written "The Sphinx: Chinning on some sandbags for protection from bombs." A Google search on: Sphinx sandbags - will result in some articles about how the Sphinx was protected during the war; the only image I was able to find to compare with my grandfather's, was this one.
I like how my grandfather made it an action the Sphinx had chosen to do to protect itself.