The manufacturer has gone to the trouble of giving the yeast all it needs for several divisions (this is for Dry yeast only). That's why you could pitch dry yeast in unaerated wort and it would still do fine (provided you don't underpitch). However, if you use the cake
from a dry yeast ferment you'd have to aerate it just as you would liquid yeast.
There was a great explanation of it on the Danstar site but I can't find it right now. Might want to peruse the site.
Here's something sort of related from the Danstar FAQ:
Q: I always aerate my wort when using liquid yeast. Do I need to aerate the wort before pitching dry yeast?
A: No, there is no need to aerate the wort but it does not harm the yeast either. During its aerobic production, dry yeast accumulates sufficient amounts of unsaturated fatty acids and sterols to produce enough biomass in the first stage of fermentation. The only reason to aerate the wort when using wet yeast is to provide the yeast with oxygen so that it can produce sterols and unsaturated fatty acids which are important parts of the cell membrane and therefore essential for biomass production.