Answer: To prevent Absorptive Atelectasis (Hunter Lung) from breathing pure oxygen long term.
Not the reasons below (as suggested by previous answers)
Oxygen toxicity (correlates with O2 partial pressure, not total pressure)
Decompression sickness (made worse by nitrogen, not better)
Fire risk (related to O2 partial pressure, not total pressure)
Thermal convection (No thermal convection in microgravity)
Air cycling ?
Astronaut overheating ?
Biology experiments ?
Human outgassing (Farts are up to 90% swallowed nitrogen)
In the early 1950s, in UK aviation medicine, the condition atelectasis (lung tissue collapse) was given the name "Hunter lung" due to its prevalence in pilots of the transonic fighter jet, the Hawker Hunter, which used a 100% oxygen supply.
Atelectasis also develops in 75–90% of people undergoing general anesthesia for a surgical procedure. (Atelectasis is due to the high concentration of oxygen in the anesthetic gas mix.)
https://en.wikipedia.org/wiki/Atelectasis.
The cause of absorptive atelectasis is (a usually temporary) blockage of small airways by secretions. If the alveoli is filled with pure oxygen, the oxygen peripheral to the blockage is absorbed and that section of lung collapses. Surface tension acts to prevent re-expansion of those air sacs once the blockage is cleared. The longer high concentration of oxygen is breathed, the larger portion of the lung tissue suffers atelectasis. Hours (EVA) or days (Apollo) of pure oxygen are tolerated, but weeks or months (Skylab, ISS) would be problematic.
Alveoli are microscopic (200-500 microns) air sacs. Since they are wet, surface tension treats them like bubbles and “tries” to collapse them (https://en.wikipedia.org/wiki/Pulmonary_surfactant . Usually, surface tension is opposed by surfactant effects or gas pressure. Otherwise, the tension would rise towards infinity as gas diffused out of the alveoli. Once alveoli have collapsed, this high surface tension prevents re-expansion.
Oxygen is very soluble in blood due to the carrying capacity of hemoglobin. If an alveoli’s airway is blocked, oxygen rapidly diffuses out of the alveoli into the blood and the alveoli collapses. Nitrogen is much less soluble in blood, so it remains in the alveoli and prevents it from collapsing. When secretions are cleared (like from a good cough), the alveoli re-expands with oxygen-containing air.
In Respiratory Medicine, nitrogen is sometimes referred to as “the skeleton of the lungs” since it prevents atelectasis.