How could I, briefly, explain why nitrogen fixaton is essential to plants and how this process works?

If you can plz suplie a good answer, I have been stuck on this all day and it will help me greatly.How could I, briefly, explain why nitrogen fixaton is essential to plants and how this process works?
In the atmosphere, nitrogen is diatomic (N2). N2 is very stable and very unusable. Plants are not able to supply enough energy to break the bonds so they rely on bacteria with special enzymes to ';fix'; the N2 by breaking it down metabolicly into more usable forms of nitrogen (NO2, NO3, NH3).How could I, briefly, explain why nitrogen fixaton is essential to plants and how this process works?
helpful for leguminous plants who take nitrates frm soil...