Because they wanted to have a positive pressure inside the spacecraft. Apollo operated at about 5 PSI in space when there was a vacuum outside, but when testing at sea level the outside pressure was already 14.7 PSI. To get a positive pressure they pumped the CM up to something like 16 or 17 PSI (I don't recall the exact number).

I believe all the Mercury/Gemini/Apollo missions were launched with the spacecraft pumped up to positive pressure. After they attained orbit the atmosphere would leak away until the 5 PSI operational pressure was reached. After Apollo 1, the subsequent missions used an oxygen/nitrogen atmosphere on the launch pad, but pure oxygen in space after this initial atmosphere leaked away.