I was wondering if someone could explain to me the purpose behind an oil breather. I saw one of these that looks like a mini air filter that apparently replaces the oil filler cap on the valve cover. Does this improve engine performance in anyway or is it hype?

On one valve cover is a breather/filter. On the other is a PVC valve that attaches to the base of the carb. What this does is pull air through the block to keep fumes from building up inside the motor. The breather/filter allows fresh air to be pulled into the motor while the PVC valve sucks the fumes into the manifold and out with the exhaust.