Originally Posted by beerocd
When you're out of primaries, and you need to brew but you're not ready to bottle. Then you secondary.
Edit: oooh lucky post - 333!
I actually just wanted to post in the "Beginner's Beer Brewing Forum" so I could feel like I hit a milestone....
But honestly, the pros will tell you to not rack to a secondary. Your LHBS (local homebrew shop) sold you a secondary because that's an extra 30$ for a cool looking 5 gal carboy. It will come in handy later because of what I've quoted from beerocd. I'm not just quoting it because it's funny, I'm quoting it because that's exactly when I rack to a secondary.
By pros, I mean the guys that do podcasts/radio shows at http://www.thebrewingnetwork.com/
A few times I've heard them say that racking to secondaries is just asking for trouble if you don't have a lot of experience with sanitization.
For my personal purposes, the only time I rack to a secondary is when I REALLY need a carboy and they're all taken. I have that damn 5 gallon carboy that my LHBS guy sold me so hey, might as well make some use from it. Folks might say "rack when you want a REALLY clear beer". But the dudes from the above will say that your racking to a secondary will result in yeast and sediment being agitated that had settled to the bottom of your primary.
The only argument I think is worth much is getting your beer off your trub for lagering (aging.... again find a show on the above link about what that is). And that only if you bring all the crap from your boil kettle into your primary.
In any case, I'm no where near most of the guys here in brew-awesomeness. But here's a for sure, the crap I've brewed has been very good and has been adjudicated to be excellent in some local competitions. Not world class worshipped stuff mind you... In any case, don't worry too much man. Unless you really screwed up (like dumping hamburger in your primary or something) you are fine to leave it in your primary.