Why is open heart surgery necessary?

Last resort. Heart surgery is typically held in reserve for patients with problems that are life-threatening that cannot be fixed with medications or fixed with percutaneous procedures performed in the cardiac catheterization laboratory.
Important treatment. Heart surgery is just one treatment arm in the management of heart disease. Certain diseases are known to be treated more effectively with surgery. There are many cases where medications or balloons/stents are inadequate to treat. In several diseases heart surgery is the "gold standard" that other procedures try to compare to.