To secure a flat or prefold diaper under a waterproof cover in the past parents had to use pins. These days there are other choices like the Snappi diaper fastener. Though not necessary, using a diaper fastener with flats or prefolds allows you to get a custom fit for the baby and reduces the amount of poop that may get on the inside of the diaper cover. In theory, that means you can reuse the cover more times before having to wash it. While there is a learning curve to fastening flats and prefolds most parents find it well worth the effort!