[AgentME]

Solar wind strips that atmosphere very slowly over geological timescales. If we terraform Mars, then we probably have the technology to replenish it faster than that loss.

[pasbesoin]

Geomagnetism also keeps your genetic change (via irradiation) from exceeding your tolerance. Relevant within single and generational lifespans.

I.e. Breathing is of limited utility, if the solar weather still gives you cancer.