Most individuals performing cardio are using it as a way to burn off excess calories and since you are moving the body, it is going to increase the need for energy. Some forms are slightly better when strictly speaking of fat loss but all cardio, regardless of form will burn off calories. Since fat loss does depend on calories burned versus calories consumed it is a step in the right direction.
The reason to do cardio does not end with fat loss though; there are a wide variety of health benefits you receive from a regular cardio program.
IMPROVED HEART HEALTH
The first one is an improved condition of your heart. Your heart is a muscle just like any other and in order for it to become strong it must be worked. If you fail to work it, it will weaken over time and this can cause a variety of negative health effects.
By getting the heart pumping at a faster rate on a regular basis you will keep it in shape and healthy. Too many people are getting winded just performing simple exercises such as walking up the stairs and the primary reason for this is because they are neglecting to work their heart muscle.
Another reason to perform cardio is for its effects on the metabolism. Along with speeding up your heart rate, cardiovascular exercise also increases the rate of various other processes in the body, also known as your metabolism.
Generally speaking, the more intense the cardio session, the more noticeable increase you will see with regards to your metabolic rate. Intense interval sprints (also known as HIIT) increase the metabolism; the highest with a process called EPOC (Excess Post-exercise Oxygen Consumption). An increased metabolism means an easier time maintaining your weight (or losing weight as the case may be).
IMPROVED HORMONAL PROFILE
Performing cardiovascular exercise also changes the hormonal profile in your body considerably. It releases 'feel good' hormones that will help ease symptoms of depression and fatigue as well as releasing hormones that decrease the appetite.
Individuals who partake in regular cardio exercise often have a much more positive outlook on life simply because they are getting the stress-relief benefits from these hormones.
IMPROVED RECOVERY ABILITY
Certain types of cardio exercise, usually lower, more moderately paced forms, can decrease your recovery time too. If you have just performed a hard session in the gym, hopping on the treadmill for a walk or light jog will help to remove some of the by-products that were created during the lifting session.
This will help to reduce your DOMS (delayed onset of muscle soreness) and help bring more oxygen rich blood to the muscle tissue improving in the repair and rebuilding process. To you, this translates to your being able to get back into the gym quicker and work the muscles again.
Building muscle mass is a combination of an overloading stimulus and sufficient rest to allow the muscle to heal itself. If you skew this balance either direction, either working out too much or providing too much rest in between, you aren't going to get optimal results.
The more frequently you are able to work a muscle though (assuming full recovery has been achieved) the faster you will add additional new muscle. Cardio helps you do this. Just don't take this too far as excess cardio or cardio done at such a high intensity that it places additional strain on the muscles is going to actually hinder recovery rather than aid it.
MANAGEMENT OF DIABETES
Lastly, for those who have diabetes, cardiovascular exercise helps them manage this condition. By performing the exercise you will increase your muscle's ability to utilize glucose. Those who exercise regularly tend to have better control of their blood sugars and do not see as many blood sugar swings as those who don't. For diabetes this is increasingly important as they are extremely sensitive to changes in blood sugar levels.
source : bodybuilding.com