I think the idea was to purge nitrogen from the bloodstream so there would not be adverse effects from unplanned pressure changes. Cabin pressure is lowered to 5 psi during ascent which causes nitrogen to come out of solution. In hindsight it would have been better to use an air-like mixture during launch and ascent with gradual replacement with pure oxygen as the mission progressed.
The use of high pressure O2 (16 psi IIRC) was to keep dust and moisture out of the capsule during ground operations
During flight cabin pressure would have been bled down to
3.5 psi for mission