"Yeast need oxygen to make good beer. The problem is that after boiling for an hour the wort is void of oxygen. Fermentations with low levels of oxygen will produce yeast that start slower, have sluggish ferments, and are more likely to poop-out and stop fermenting. On the contrary, fermentations that have been infused with oxygen will have healthier yeast that start faster, ferment faster, have a greater tolerance to alcohol, produce beers with less residual sugar, and produce better overall flavors. " Source morebeer.com

Here's a good read on the subject, if you want more information.
http://www.liquidbread.com/whay-oxyn.html