When people used to heat their homes with coal furnaces, there would be soot left on the walls. If they had wallpaper, then the soot couldn't be washed off. Play-doh was originally marketed as a cleaner for wallpaper. Back then it was called Kutol Wall Cleaner.
When people stopped using coal furnaces in their homes and started using washable wallpaper, the wall cleaner started being sold as a modeling compound instead.