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To acheive full thrust of the first stage F-1 engines of the Saturn V only liquid oxygen should be pumped into the combustion chambers. A mix of gaseous and liquid oxygen would reduce the desired mass flow of oxygen to the engines and may damage or even destroy the oxygen pumps. A reduced mass flow of oxygen would reduce thrust and endanger a successful liftoff.
But the LOX lines through the fuel tank had a bad ratio of surface to volume, much worse than the large oxygen tank. The heat flow to the LOX lines would cause the LOX inside to boil heavily.
By bubbling of cold gaseous helium through the LOX lines and tank the LOX was cooled before launch below the boiling temperature. The gaseous helium and oxygen at the top of the tank are exhausted through umbilicals and the evaporated LOX is replaced by toping of more LOX from the large tanks at the launch pad.
Just before ignition the helium bubbling is finished and the very cold LOX within the lines and tank does not boil as long as its temperature is below the boiling point. So the oxygen pumps and combustion chambers could be fed with pure liquid oxygen free from bubbles.