One of the more popular things enclosed with letters is photographs. The photos may be of the letter writer or even of their family. Or, it could be a picture taken on holiday or in the surrounding area. Maybe it is something funny they've come across. But, perhaps the 6 x 4 (inch) format is too big for letters and may make the letters too heavy or thick. I don't have a working colour printer at the moment, but if I did, I could print some small pictures on the paper I use for letter writing. However, most of the paper I use is A5, and is perhaps not all that printer friendly.
So, I was pleased to see a photo printing machine in a local supermarket that would allow pictures submitted on various digital media (USB, memory card, plug your device into the machine for image transfer....) to be printed. I can be stingy, so rather than get a small amount printed for a higher cost each, I go for gazillions (well) for the cheaper cost per print. But they come out 6 x 4 or can go for the larger size of 7 x 5.