In the course of our long evolutionary history, iron has not always been abundant in our food; for this reason, as well as its critical necessity, our bodies have evolved mechanisms to grab iron and hold on to it. But we have not evolved any way of getting rid of it.
When humans are growing, they require plenty of iron for their development, but after maturity, the iron accumulates, often to high enough levels to damage cells and lead to disease.
Iron is a reactive element. When exposed to air and water, it rusts, and when inside our bodies, it can react with components of our cells — lipids, proteins, cell walls — and damage them. That’s how it leads to illness and premature aging.
Reducing excess iron may have been the main benefit of the ancient practice of bloodletting.
You know, medicine is not an exact science, but we are learning all the time.