What is the purpose of the natural openings in the coronary arteries?

Blood flow. The coronary arteris originate with an os or opening which directs blood from the aorta into the coronaries and then into capillaries to provide the heart muscle with blood.
Coronary ostia?q. Not exactly sure what you're asking but each coronary artery has a normal opening in the root of the aorta which allows it to fill with blood. These openings are called the "ostia" (singular=ostium).