Because they replace Diaper Pins! Snappis are the most popular cloth diaper fasteners in the market. Over 80 million units have been sold worldwide, and there is much trust in the Snappi name.
Additionally, people really love these fasteners when using prefolds for night time diapering. Snappis are easy to use even half-asleep in the middle of the night (after your husband asks for YOUR help when it's HIS turn). They take less than 20 seconds to secure, even with a wiggling baby. Obviously diaper pins are hard to handle in this situation.