Nitric Oxide supplements are designed to enhance blood flow, oxygen and nutrient delivery, which helps muscles become larger when subject to stress.
Nitric Oxide: More Information
Without this oxygen and nutrition, your body becomes vulnerable to fatigue and inefficient recovery. It's also Nitric Oxide that is mainly responsible for inducing the "pump" during workouts.
Nitric oxide booster products can be useful for people looking to get huge, lean out, or simply increase cardiovascular endurance. Essentially, anyone actively training can find a use for this product.