The purpose is to prevent buildup of harmful gasses and VOCs (everything from CO2 to CO to regular out-gassing of furnitures, etc) that can become dangerous if not cycled regularly.
2120: The environment is all full of poisonous gasses from the people of 2020. Each house needs its own air purifier to make the outdoor air safe to breathe.
> the first Cordis hotel on mainland China boasts something that is genuinely rare in big Chinese cities: clean indoor air.
https://www.theguardian.com/cities/2018/mar/27/china-clean-a...
That's the case in much of the world already... I live in area where there's tons of smog in the colder months and consider installing a system for bringing in filtered air from the outside, so that I won't have to open windows for the 6+ months smog period.
One reason is efficiency. In the Bay Area, it's common when the daytime high temperature is 80–90˚F for the nighttime low to be 50–60˚F. You can set your thermostat to 76˚F until sunset, then bring in outside air to cool down to 68˚F or below by sunrise. You get cool air when you're trying to sleep and avoid running the AC in the night or morning.
I'm getting my furnace & AC replaced today and was sad to learn I couldn't get one of these. The fresh air intake has to be at least 10' from the furnace flue vent, and that isn't practical with my home's design.
But there is also typically fresh air requirements to bring in some percentage of fresh air.
And lastly the HRV/ERV mentioned in the other comment is typically more a residential thing from what I've seen but that's totally a thing I want in my next house.
So some parts of construction techniques and building materials have improved significantly. However, when comparing the framing lumber used today to what was used in the 60s, I'm shocked that new construction doesn't fall over the first time the wolf huffs and puffs.