How came they adopted the idea of pure O2, that also lead to the Apollo 1 disaster? From a layman point of view it is so absurd.Weight reduction
I was waiting for an expert to answer and who might have some numbers on how much it saves in terms of launch mass. Anyway, for each kg of command module returning to Earth it took more than 500 kg of Saturn V launch mass! So, reducing the mass of this module really made the difference.
The atmosphere at sea level corresponds to a water column 10m/30ft tall. On the launch pad that is no issue given that internal and external pressure are in equilibrium. In space there is no outside pressure and so by taking out the nitrogen the internal pressure goes down to about one third. Hence less load on the structure.
BTW, this is also done in commercial airliners by lowering the cabin pressure when climbing to cruising altitude - although without changing the oxygen/nitrogen ratio. That means less O2 for the passengers than on sea level which is acceptable for sitting in a chair but not desirable when performing work in space for extended periods of time.