Why do we put nitrogen fertilizer do to help with plant growth even though there is so much nitrogen in the air

Many plants cannot extract nitrogen from the air.

Explanation:

Many plants require nitrogen for proper growth.  However, only a few types of plants, mainly legumes (Plants with nodules that house nitrogen-fixing bacteria) can extract nitrogen from air.  As a result, most commercial farms require nitrogen fertilizer for proper yield rates.

which of what? i am confused