A few years ago I was given an old suitcase, full of old photographs. The suitcase had come from my wife's grandmother's flat, when it was being cleared out after her death, and it was given to me as I am the designated family historian and keeper of old photos. There were hundreds of them in the suitcase, along with a couple of books and some holy pictures. Do you remember holy pictures? Well, the great thing about these holy pictures was that they had dedications on the back, with dates and "love and kisses from Alice".
The photos were a different matter. In all, I think only two had any form of identification, and all the people who might have known those pictured were themselves dead. What is the poor designated family historian to do with them? Sensibly, the only thing is to bin them, but as I am also the designated keeper of old photos, I have put them all carefully back in the box, for some future keeper of old photos to worry about.
Nowadays of course we have thousands of photos – I average 2,000 a year, after deleting rubbish and duplicates – and few of them get printed, so there can be no writing on the back. But they are digital! Your camera will have recorded the date and time that you pressed the shutter, along with all the camera settings that you used, and some cameras will even have recorded the GPS co-ordinates. So all you have to do, to be able to answer "Who is that?" as your memory fades, is use your software to add a caption, which will be stored in the EXIF data with all the other details. You owe it to posterity to do it.