Question

Why Hb is more affinity to oxygen

Answer 1

I think it should increase for better oxygen transfer and uptake from air. The concentrtion of oxygen in the atmosphere decreases with height. Hence, if the haemoglobin binding increases, we will be able to draw more oxygen from the air and transport it to the cells.

Answer 2

We need oxygen. We take all gases from air.
If Hb did not have affinity with oxygen, any gas could bind with it.
Not good if your lungs get sulphur dioxide instead of oxygen.