Folding cameras were immensely popular: Kodak made them at the turn of the century, and they were still being made into the 1960s (e.g. the Kodak Retinas). They were the next step up from a box camera in quality: they typically had better lenses that could be focused, and they could take the same quality leaf shutters that the high-end cameras had. Yet they were relatively trim and compact. But as the amateur market shifted from roll film to 35mm and the japanese released more smaller, lighter-weight cameras, the folders lost their niche to the rangefinders.
The Agfa Billy is a testament to the popularity of the model. It was first manufactured in 1928 and ran through 1960 (with some tweaks along the way)—an incredible run for a camera. Like most folders it was cheap, resonably light and compact, but still a decent machine. They came with a variety of lenses and shutters, mostly so-so.
I think I paid $5—maybe $10 for it at yard sale. The most interesting thing is that like other German folders, it used 120 film which is still in production; so you can still take photos with it today. Kodak folders almost always take 620 film which has long been discontinued; so if you want to use them, you either have to modify the camera or buy re-spooled film from specialty film companies.
Camera manual: Orphan Cameras.com