Actually the hose to the air cleaner is to draw IN filtered air, just like the carb does. On a healthy running motor, you can pull the cap and feel it drawing in air. After changing to an aftermarket air filter a lot of times there is no hose hookup provision. So you would chunk the hose and change the cap for one with a built in air filter.
At some point your engine will wear enough to where there is more blowby than PCV vacuum. Then indeed that hose will blow the blue smoke into the carburetor. If you had the filtered cap, it would emit a blue cloud and soon start clogging up with oil mist.
If your engine is a beater, you'd prefer that mess to be sucked into the carb so you can pretend it's not THAT worn out. At least until it starts blowing blue smoke out the tailpipe.