Back before Datsuns really hit the world market, car engines suffered form oil leaks & from cracase sludge. This sludge was wretched stuff & in engines that were little used, & even less serviced, it could build up into incredible thickness throughout an engine & it was awfull.

From the sixties onward, these problems dissapeared because of a few simple things.
Better oil, & PCV systems. Improved seals also helped.

Sludge was formed when the condensation that formed in the crankcase was mixed with combustion soot, & sulphur, & dirt & ash, most of which are combustion by-products. When a positive draught was introduced into the crankcase, the vapours of water, fuel, & any material that was suspended in the crankcase gasses was drawn into the manifold, burnt, then passed out the exhaust, then sludge all but totally vanished from engines.
Improved oils held more solid matter in suspension better, & detergents in the oil helped to keep everything cleaner. At oil change time, much more of the crud in the engine was flushed out into the oil catch can The actual amount of fumes that are drawn into the combustion chamber is quite small if it is done continuously in a running engine.

The reduced pressure in the crankcase tended to pull the seals onto the rotating crankshaft & helped to prevent oil leakage. The hose from the aircleaner to the rocker simply provides clean filtered air to the inside of your engine.

Disable this proven system at your engines perril.
The Positive Crankcase Valve simply controls the amount of air that passes into the manifold at diferent vacuum levels, & prevents a backfire from reaching the crankcase. It's a good thing for your street engine.