There is no performance gain by doing it. There is no danger in it either. It is part of the PCV system. This is basically how it works:
Our stock PCV system is a closed version. Air is drawn in through the tube that connects to the intake and into the front valve cover. This vent allows air in the crankcase to be sucked out through the PCV on the rear valve cover. Without this vent in front, air could not escape out the back. When you install a breather in place of the hose from the intake, you basically turn the PCV system into an Open system, which is somewhat less effective at controlling pollution, since gases that occasionally back up through the front valve cover are now released into the atmosphere, instead of going back into the intake. Granted, this does not occur often, as most of the time air is being sucked into the front valve cover, not out, and the effects of the gas release are probably negligible. With the stock setup, it is possible for small amount of oil to travel the tube and end up in the intake. A breather will solve this problem if you are having it, while making the car slightly less environmentally friendly I highly doubt it is enough to make you fail an emissions test or anything. Hope this answers your question
Whoa, very in-depth. I just figured by not recycling the air and its contents, I'd have less of that fun buildup in the TB in the long run. Plus, hey, it'd make me look different, I guess. And I'm not overly concerned about what gasses would escape, there's far worse polluters out there.
You can buy the breathers from Autozone. They have a section dedicated to the APC product and thats where I got mine.
I personally think they look cool and much better than a hose going into the intake.
Check them out, they cost about 7-9 dollars in the US.