After boiling the wort, aeration is a critical step in a healthy fermentation. The boiling process removes dissolved oxygen which is necessary for yeast growth. This is the only phase of brewing in which the wort should be exposed to oxygen.

Adding oxygen to the cooled wort aids in yeast metabolism and decreases the lag time between yeast pitching and the noticeable start of fermentation. Without aeration the yeast struggle and fermentation starts very slowly. This is not desirable because of the stress it puts the yeast under. Decreasing lag time also decreases the chance of contamination by bacteria which may be growing in your sugary wort. The rapid start of fermentation makes conditions unpleasant for these bacteria, and kills them off.

Because aeration has a direct link to yeast metabolism, aeration or lack thereof, can have an impact on the final flavor of the beer. (1)