As you've noted, it is very common for cancer treatment, though also used in cases of heart diseases and other blood related ailments. It's considered something of a myth. In other parts of the body, besides the heart, there are a variety of blood vessels such that when the main one slowly closes off due to vascular disease, there are alternate pathways that enlarge and handle some of the flow.
The primary example of this is in the leg, where disease of the superficial femoral artery (the main supply down the leg) leads to the enlargement of a nest of blood vessels which flow through the buttock and deep thigh. Termed "collateralization", this can clearly be seen with angiography (blood vessel X-rays) in cases where people have problems with the blood flow to the leg.
Even when the collateral blood flow to the leg is enhanced over time, it does not meet the demands of the muscles if they are taxed. People with this type of vascular disease suffer from "claudication" which is aching pain in the muscles, thus prohibiting them from walking any great distance.
Unlike the leg, with large muscle groups supplied by multiple large and small blood vessels, the heart has only two main blood supplies which are only several millimeters in diameter at their origins. The right and left coronary artery branch directly from the aorta right after it leaves the heart, and angle directly back to feed the heart muscle. Each region of the heart muscle has arterial supply from branches of one of these two vessels, and there is only a small area of overlap. Because of this minimal amount of shared duty, there isn't much opportunity for collateralization. No other blood vessels are connected to the heart and thus no other supplies are capable of increasing the blood flow to it.
Unfortunately for the heart, the kinds of things that promote vascular enlargement also promote ischemia. For example, in order to get the leg to utilize collateral blood supplies, we have the patient walk until it hurts, and then walk some more! In the case of the heart, taxing its output like this would precipitate a heart attack, because the hearts only response to increased demand in this case would be to increase its rate, and if the rate goes up, so does the oxygen demand.
Unfortunately, the nature of coronary artery disease is that the amount of blood flow to the heart muscle is fixed and cannot increase, therefore the demand quickly exceeds the supply, and without sufficient oxygen, the muscle suffers gravely!
Coronary artery disease, if discovered while there is a minimal amount of blockage, can be managed with lifestyle changes and medicines such as aspirin which decrease the likelihood of sudden tiny clots blocking the flow. If it is discovered when there are more significant blockages, then something needs to be done. The most durable method for increasing blood flow to the heart and keeping it going is surgical bypass. A less invasive method, but one which doesn't statistically last as long is called angioplasty and stenting. This second procedure involves an X-ray guided, balloon tipped tube placed into the coronary arteries and then inflated to streatch them open. A spring loaded metal cage (stent) is then deployed inside the narrow spot to hold it open.
More important than any treatment for coronary artery disease is DISEASE PREVENTION. Once someone has developed vascular disease, they will never have normal arteries again. A healthy lifestyle with exercise, reasonable diet, and the avoidance of smoking is by far the best medicine!
