From a yeast cells point of view, its purpose in life is to grow, eat, and reproduce. Yeast can do all this with or without oxygen, but using oxygen makes the processes easier for the cell. Yeast use oxygen in the biosynthesis of the compounds that make up their cell membranes that allow them to process sugars for food and grow. Being able to process food and grow more efficiently allows them to reproduce more effectively also. Without oxygen, yeast cannot reproduce as fast. Therefore, to ensure a good fermentation, we need to provide the yeast with sufficient oxygen to allow them to grow quickly and reproduce when they are first pitched to the fermenter. Once they have reproduced to sufficient numbers, we can let them get on with turning our wort into beer.