I assume you are talking about piston engines here. The movement of the pistons and the clearance between the piston rings and the cylinders allows some of the presure created by the explosions in the cylinders to pass into the crankcase. If the crankcase was a sealed chamber then the seals around the crankshaft would blow out dumping the engine's life blood, oil, over the side. This would result in a large silecence from the engine. The breather is the vent to release this pressure.
This keeps everybody happy
