What is enriched air and why would divers dive with enriched air?

Enriched air, also known as nitrox, is one kind of air with more than 21% oxygen. 21% oxygen is the normal air we breathe on the earth. The primary benefit of using enriched air is to extend the diver's bottom time, and reduce the need to push to no decompression limit. This advantage is really beneficial in deeper diving. 
For example, at 80 feet of depth, normal air can last around 30 min, while enriched air of 32% oxygen can last for 45 min and 36% can last for 55 min. 
While enriched air is really beneficial in extending bottom time, it is important that divers get trained to use it before diving with enriched air.