They make the lining of the uterus thin so the egg can't stick. It's easy for most women to take one pill a day and not have to worry about getting pregnant. Current low dose pills rarely have side effects. You can choose exactly when to have your period, or when not to have it. PMS is often less.
Blood loss is reduced, so the chance of iron deficiency anemia is much less.
They reduce the probability of developing endometriosis. They make your bones stronger. Your breasts might get larger. Your complexion will improve. If this is important to you, tell your doctor, so you can get the pill that's best for this.