Around 1964 automobile manufacturers started adding a PCV valve to their engines. This was primarily motivated to reduce pollution caused by venting the crankcase vapors directly into the atmosphere. The source of these vapors is combustion gasses leaking past the piston rings. Earlier engines simply had a breather from the crankcase as you mentioned in your post.
The PCV valve is more complicated than most people realize. One end is connected to your crankcase, usually via a hole in the valve cover. The other end is connected to your intake manifold downstream of the carb. With the engine off, the PCV valve is closed. After starting, with the engine idling, the intake manifold vacuum is high. The PCV valve is "pulled" open by this vacuum a small amount to allow the crankcase vapors to be sucked into the intake manifold and reburned in the engine. As the throttle is opened and a load is placed on the engine, the vacuum drops. Since this happens at higher engine speeds, there is more blow by in the crankcase. At this lower vacuum level, the PCV will open more and allow the added gasses to be passed back into the engine. The PCV operates in three states, closed, lightly open, and fully open.
Even though this was originally conceived to control emissions, they found that by removing crankcase vapors with the PCV, the engine stayed cleaner and longevity increased. The other benefit was with reduced crankcase pressure, oil leaks went down because the gaskets did not have to hold back as much pressure.
Here's a great video on PCV operation:
https://www.youtube.com/watch?v=pr5MZni4_Bc
I'm not a big fan of off-the-shelf PCV valves. They may not do much good if they aren't for your specific engine combination.
Here's a product that allows you to "tune" the PCV valve for your engine.
http://mewagner.com/?p=444