Presumably for whatever reason it worked well for us to develop as we did down here? Stops everything from exploding, dissolves all the other gases quite nicely? What other gas would you use?
If you didn't have nitrogen you would need another inert gas to make up 80% (or whatever) of the atmosphere you were going to make.
Being in a pure oxygen environment does not have a long term impact on animals… if the pure O2 environment is 0.21 atm oxygen. There’s a long history of biological experimentation done by space agencies on this, and the Apollo missions operated on a 0.2 atm pressure for the duration of their missions. Apollo 1 famously had trouble not because it was pure O2, but because it was 1.2atm O2.
There is no reason you need backfill inert gasses to have 1.0 atmospheres.
The vast majority of the biomass on earth, including about half of the fixed nitrogen in your body was synthesized industrially via the Haber-Bosch process over the last 100 years.
Aside from those nitrogen fixing bacteria whose output we have already surpassed with industrial processes (artificial fertilizer), inert nitrogen plays no role in biological metabolism. It is not needed, at all.
Mars colonists can import nitrogen, or mine fixed nitrogen from the few deposits that are known to exist. It doesn’t need a massive atmospheric reservoir.