Nitrogen is, as far as our biology is concerned, effectively inert, we don’t really do anything with it, it more or less just goes in and out of our lungs. For most practical purposes under normal atmospheric conditions it could pretty much be replaced with just about any nonpoisonous gas. As far as your body is concerned that part of the atmosphere might as well be helium, and in fact for certain deep sea diving applications and such we do replace some or all of the gas mix with things like helium because nitrogen will sort of dissolve into your bleed at high pressure, which causes issues when you start to resurface and it creates nitrogen bubbles in your blood (known as “the bends” or more technically as “decompression sickness”) and those other gasses don’t dissolve into our blood as readily.

Pretty much as long as oxygen is at the right percentage, your body doesn’t care what the rest of the gas mix is as long as it’s not outright poisonous.

Now there could be issues for nitroget-fixing bacteria that do use atmospheric nitrogen and convert it into other nitrogen compounds that are absolutely necessary for plants and such to grow (and by extension for us and everything else that eats those plants, or eats things that eat plants) to live, and I’ll be honest, I have no idea at what level of atmospheric nitrogen that would start to be a problem, and unless we want to start growing crops in underwater domes it’s probably not something we ever really need to worry about on earth, nitrogen is very plentiful in our atmosphere. It could possibly be something worth investing for long-term space exploration and such, but we’re not quite there yet.