Can you please tell me why it'snecessary to limit o2 flow rate with people with copd?

Depends. In some patients with severe copd, the drive to breathe depends on the oxygen level and not the carbon dioxide level. Normally, the level of carbon dioxide in the blood is a very potent stimulus for breathing. But in people with severe copd, this stimulus is blunted because the carbon dioxide level is high for a long time. Thus, the body depends on oxygen levels. This is not very common, though.