It won't! There are many benefits associated with cardio and aerobic training, but also some negatives. The benefits of cardio is that it can lower your resting heart rate, lower blood pressure, keep your brain young by increasing circulation to the brain and aids in detoxification by stimulating the lymphatic system.
But traditional, long distance cardio training decreases testosterone levels, decreases the immune system post exercise, and raises stress hormone levels (cortisol). Increased levels of cortisol stimulate the appetite, can increase fat storing and slow down or inhibit exercise recovery.