Yeast needs oxygen to reproduce so we make sure there is plenty of oxygen in there at the start. The yeast will quickly scavenge all the oxygen in the container, multiply, and then start making beer. At this point we want to limit oxygen as once the yeast run out of food, they will stop scavenging oxygen and any left over will oxidize your beer.
If you are bulk aging your beer for months instead of weeks, you might want to move it to a container with zero headspace. There is no fermentation going on at this point. True 'secondary' fermentation doesn't start until you add more sugar, like priming sugar at bottling time. That will cause the yeast to re-activate, scavenge any oxygen in the bottle, and carbonate the bottle.