If you leave it like it is, just the hose, it won't do any harm but it might leave a little mess wherever you put the opening at. It is used to release any gases that the oil builds up in the engine. I'd get a aftermarket breather at autozone or some other auto parts store and put it on the end of the hose.
Besides, it looks cool...
...its the round red thing next to the airbox. BTW, this is not my engine, its SleeperZ's and you can find out how to make the intake by going to the related sticky in the main index of Zetec Performance section if you want to do something more "exotic" to your intake.
