Well, let's see. It has 7 protons in the nucleus of every atom,
and it forms 78% of the entire atmosphere on Earth. No other
gas can make that statement.
The second question bothers me. Things in nature don't have
functions or purposes, aside from the divine plan. They just ARE.
About four fifths of our atmosphere is nitrogen. So a whole world
of fruits, vegetables, trees, and millions of other growing things that
use nitrates in the soil evolved to draw their life from nitrogen. If the
nitrogen suddenly disappeared, they would too, so I guess you could
say that the main 'function' of nitrogen is to support plant growth.