there is a tube connecting the crankcase to the intake and it gets its air from there. but if you get an intake w/o a hole for the tubing, then you can use a breather. i am using one in my mom's caravan since i put in a custom CAI. haha. not for the legend tho, i just used the one from procarparts.
Basically it's a air filter. Made to fit on the PCV valve. Typically, as JJJ784 said, there is a hose connecting the intake track to the PCV valve. Weather (damn, my mind's gone today) you have a cheap non-carb intake system that doesn't have that or you built one intake system yourself and didn't make a connector for that. but when it comes time for smog, it's a no-no. With the PCV valve, not only does it "breath in" air, it also releases burned motor oil gasses to be burned in the combustion chanber.