You want a negative pressure in your crank-case. Alot of drag racing guys use vacuum pumps to pull as much air out of their crank-cases they can to keep a vacuum, which in turn improves piston ring seal and lengthens the life of your internal parts. I believe that's why alot of breather tubes run from the breather to inside the air cleaner. As the carb takes in air through the air cleaner, the air moving by the breather tube creates a suction.