Yeast is an interesting microorganism because it can grow with oxygen (aerobic growth) or without oxygen (anaerobic growth). The difference is where it derives its energy; without oxygen, yeast must convert a nutrition source such as sugar into energy, a process which is inefficient.
Where oxygen is present, the yeast can concentrate on growing and maintaining its cells, and thus produce little waste (alcohol and carbon dioxide). This process is faster and more efficient.
Without oxygen, the yeast uses its nutrition to produce more waste, allowing less cellular growth. This process is slower and much more inefficient for the yeast itself but is useful for rising bread and fermenting alcohol.
In an aerobic situation, the yeast can replicate itself quickly, which is useful for manufacturing, although care must be taken to limit the amounts of sugar or other nutrition so as not to contaminate the yeast with alcohol. In an anaerobic situation, the yeast affects other compounds, which is more useful for cooking.