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?
Like, without that inert gas there would have to be something else. Being in a pure oxygen environment for a prolonged period has health impacts to many animals including us. Anything even remotely flammable, like paper, wood, sugar, would burn uncontrollably with the slightest spark.
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.
For the most part, partial pressure is what matters, not absolute pressures. The only contribution inert nitrogen has to life is raising the boiling point of water. But on a frigid planet like Mars that’s not going to be an issue.
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 Nitrogen cycle [1] is a critical component of many ecological systems. Critical compounds in plants have nitrogen, and the nitrogen for these is absorbed from the soil, where it is fixed by bacteria.
The nitrogen cycle almost entirely involves fixed nitrogen, not inert N2. Nitrogen is fixed from the atmosphere slowly over time through lightning strikes and a very small number of nitrogen fixing bacteria that live symbiotically on the roots of very few plants.
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.