Oil expands when it gets hot like anything else, as well as making hazardous vapors that steam out of the crankcase when the bike is running. The engine itself needs to not be air tight so that it doesnt explode like a popcorn kernel once it gets too hot, and the gases emitted from the steaming oil are pretty ugly stuff. Cars have EGR valves and some bikes have evaporative emissions control systems to reduce these and other unburned fuel matter, recycling them into the air intake for further combustion. 

That is really just an air filter for your crankcase.

Typically in a hot rod job for a bike that would be something crucial for removing your airbox and moving to an aftermarket intake platform, as that port on the top of your block would be hanging open. Unless you were planning on a super overhaul of the VT1100T engine for the sake of pushing as much power out of it as possible, its really not going to change a thing. Its more a thing you would have to add after removing the airbox.

Edited 1 time by ineclipse22 May 23 14 6:36 AM.