A bag that enables you to carry the weight on your hips, allowing full upper body mobility while you ride. Smaller models are useful for carrying energy bars and essentials, whereas some larger bags have the capacity to contain all your kit for a full days riding.
Keeping your kit to hand is great for any activity. The added benefit of using an unutilised body part is an additional benefit of the waist bag. Whether you choose to store the essentials or full kit in your waist bag, you'll have instant access to your items.
Many waist bags are designed to carry bottles, whereas some are just standard pouches. Think about what kit you're likely to want to hand at all times and whether you will be taking any other form of bag. It's a good idea to keep at least some spares/tools and nutrition handy.