Answer:
Bicarbonate & CO2 dissolved in the blood act as buffers. They make it so the pH of the blood doesn't change much. It's important because the hemoglobin of the red blood cells changes its conformation (shape) when the pH changes. If the hemoglobin changes its shape, it won't transport oxygen, and you die...
There's a medical condition for this - 'acidosis' - it's when you're body isn't efficiently getting rid of CO2, and the blood gets too acidic to transport oxygen properly. Potentially, it's potentially a life-threatening condition.