This is a clean-air device that captures the emissions from the crankcase and condenses them in that tube you see with the plug in the end. Instead of the vapors escaping from the crank-case (via the rocker-box cover in most cases) into the open atmosphere, the fumes are contained. When nobody's looking, you can pull the plug and let the stuff escape into the open air. There's not a lot of fumes, just a microscopic amount. Maybe a little dribble now and again. But it's important in the eyes of your superiors that you manage your emissions (!) responsibly. That's what my dad always told me, anyway.
And, as an aside, running at top speed, wide-open-throttle, increases crankcase pressures and your highly-stressed engine will start blowing oil out the crankcase vent tube. This may show up as oil on your rear tire. The remedy is to slow down.
