The Apple Watch is a fairly tough device. Scratched screens represent some of the most common damage, which a case can help with. A raised edge around the display keeps the glass surface off the ground and away from small particles that can cause distracting scratches. If you have an aluminum model (as opposed to stainless steel or titanium), the metal case is also more prone to dents and dings. A case can help protect the edges that take the most abuse.