Whenever you would use a diaper pin! If using a prefold or flat cloth diaper, Snappis make sure the diaper stays in place and doesn't bunch up under the cover.
Snappis are great for our flats, preflats, and fitted diapers. Since Snappi fasteners work by gripping onto the fabric of diapers with little teeth, it will not work with fabrics that cannot be gripped like microfiber or thin gauzy weaves. Snappi fasteners work best with thick woven cotton or hemp fabrics. They also work great on our flax linen diapers!