You may already know but just to add to things, they were extremely common because machining tolerances weren’t quite there yet to make cheap reliable interchangable magazines. Even once they were starting to be designed to be interchangable soldiers would still need to find a particular magazine that works reliably in their specific weapon. Stripper clips were far cheaper to make and while not quite as fast as changing a mag in a modern firearm but once you are used to it it’s not that big of a disadvantage.