Lubrication of the crankcase (crank, camshaft, bearings, conn rod etc) occurs by means of blow-by oil through the piston ring...so oil accumulates in the crankase, and it is being pushed to everywhere in the engine....the oil has to get out somewhere, and that´s why most of the 4 stroke engines have a nipple at the bottom of the camshaft or at the backplate...Some enignes like the one you have, recirculates this oil to the intake to make them run cleaner, since the amount of oil expelled may be annoying etc...