They are engine accessory drive belts. They are attached between the engine crank shaft and the engine components like:
- Water pump
- Air conditioning compressor
- Power steering pump (in some cases)
- Cooling fan (when not electric)
Most of these separate belts were replaced by one longer drive belt called the serpentine belt. It was attached between all or most of the components listed as well as various pulleys to provide mechanical power from the engine to run them.
Prior to 1990, the belts shown were pretty ubiquitous to every car. And, like any other part, were prone to wear. Checking and replacing them was a common point of regular maintenance. More common than changing tires since your average driver could do it themselves. Almost as common as checking the oil.
These types of belts were thinner than the more robust serpentine belts. The serpentine belt in a shop would look very much like a thicker version of the belts shown. Just doubled up in the package due to their length. Some of the belts in this shot may be early serpentine belts. Their thickness would be the determine factor versus a pack of two, thinner, drive belts.
Even into the beginning of the twenty-first century, cars with regular drive belts would have been plentiful on the American road as daily drivers. Now, they are more of a Collectors item. But, even as late as 2010, I would change my own serpentine belts and tensioner pulleys.