There are a whole host of reasons for the fascination with buying and developing old rolls of used film, as the Guardian's Amelia Tait found.
It's not just a voyeuristic interest in other people's lives (though that does undoubtedly come into it) - it's also the thrill of potentially discovering something of historical, cultural or social import; it's the pleasure of "being the first person to ever see these images" (as one enthusiast Levi Bettwieser puts it); it's the desire to save personal items with sentimental value from being completely lost; it's like bringing ghosts to life (to paraphrase another fan, Jack Jewers).
As Tait points out, in the pre-digital, pre-cameraphone era "every roll ... was mystery film - most of us didn't know which pictures would come out, how we were going to look, or which shots would have a blurry finger in the corner, cutting off half the photograph". Perhaps that helps to explain why the forgotten film fans she quotes are all relatively young, in their 20s and 30s - perhaps it's a yearning for a form of excitement that we've lost with the immediacy of digital photos.
Of course, developing these films only generates more questions. Why had the rolls been discarded or neglected? Who's in the photos? Who took them, and why? Bettwieser clearly enjoys the hunt for clues: "When you're taking a picture of someone in your house, you think you're taking a picture of them but what's around them tells a greater story. You look and try and piece together a story of who those people are."
Given the last word by Tait, Bettwieser does a good job of encapsulating the value of photos generally, even if the subject matter is ostensibly trivial or meaningless or the people in front of or behind the camera have been long forgotten: "I try and look at every image I rescue as if I'm looking at it in 50 years - everything I rescue is history. People hold on to rolls of film for years and years in the back of a drawer, because we all know that pictures are history, whether it's just a birthday party or not. Pictures are our only defence against time, our only evidence, sometimes that we ever even existed."