Most people are dry hopping for aroma. The best place to do it is at the 2ndary after its off the yeast from the primary. Usually people stuff the 2ndary with hops then rack the beer over on top of the hops.
Also when you use dry hops there is a chance of infection if you do it in the primary. (Before the yeast takes hold) The secondary has active yeast and the built up alcohol the will prevent most organisms from taking over the beer.
One caution about dry hopping, don't keep it on the hops for more than a week or two otherwise it can get grassy-like flavors.
Schlenkerla is right along with the fact that when fermentation takes hold in the primary the yeast is scrubbing the hop flavor along with all the other good and bad tastes. Dry hopping is always done in the secondary do to this an the aforementioned reasons. Also adding them after primary fermentation and just befor consumption lets the hop aroma be at it's freshest.
Ditto. You want the best hop flavor possible...some people add hops at every juncture in the brewing process. I like to sanitize a fine mesh hop bag, throw my hops in, then affix the hop bag to my keg's dip tube with a hoseclamp. That way, I can secondary in the keg, age it, and when the level of beer in the keg drops low enough, the bag will be out of the beer and cannot add grassy flavor.
Maybe I should have been more clear, I fully understand the purpose of dry-hopping, and I know what it adds to the beer. My question is why can I not add it directly to the primary fermentor and keg or bottle after two weeks, rather than waiting a week, racking, dry hopping, and waiting two more weeks? Do I make sense? Does the active yeast somehow ruin the hop aroma?
I don't bother with secondarys, i leave my brews in the primary for several weeks to a month, as long as you don't throw in the hops when the initial violent ferment is going (the Co2 being produced strips out a lot of aroma compounds) on you will get plenty of hop aroma.