Actually, it is supposed to be a closed-system. The breathers included in the kit are used (or similar if you want something else.)

The kit also includes grommets to seal the breathers to the valve covers.

Serious race engines (some street engines too) use crankcase vacuum pumps (if legal per the controlling sanctioning body) to remove the air and extraneous combustion gasses, etc, from the crankcase.

This provides better ring seal, but in addition, removing the air improves horsepower, as the back sides of the pistons do not need to move air around inside the crankcase.

You think about the air above the piston (air/fuel mixture) but there is also air inside the crankcase below each piston that has to be displaced/moved around, energy being expended in the process. Also, there is less wind resistance for the crank to spin around as well.

A Pan Evac provides some of these same benefits to a much lesser degree, but still helps.

I use both the Pan Evac and PCV on the 440 in my '71 Charger and even the AMC 304 V8 in my CJ-5 Jeep.