What is so special about oxygen for respiration if there is only 21% of it in the atmosphere? Since there is about 78% of nitrogen gas in the atmosphere, how come biological organisms have not evolved by respirating with nitrogen gas?

Is there something special in the chemistry of oxygen that makes it so important about respiration over nitrogen?

If there is more nitrogen in the atmosphere, how come we never heard of any biological organisms that respirate or respirated with the use of nitrogen gas?