This should help with the basics. Essentially anything that doesn't go out the exhaust line (small leakage by the rings, stuff picked up by oil, etc.) that end up in the oil sump get sucked out through the PCV valve. The valve itself can get gunked up over time, hence why you change it.
When the engine is at idle (or low loads), the intake is a lower pressure than the manifold (the rest of the engine). So vapors from the engine can get sucked into the intake and burned. This is water vapor, hydrocarbons either from oil or unburned fuel. This is done to reduce emissions of the engine. Other cars (off-road/racing) can use a venturi tube on the exhaust to suck out the vapors from the manifold and inject them into their exhaust stream.
With a turbo system, when the engine is going decently, you now have a higher pressure on the intake than the manifold (due to a compressor). So either you raise the manifold pressure so that you can keep the same system in tact (high mani pressure vs. intake pressure to get "flow") or have it sort of "hold up" until the load/RPMs go back down to "low" and the system "reengages" to the above picture.
Essentially a catch-can will allow for water vapor to be condensed, some gases to be absorbed the water and other items to condensed (HC and the likes) due to a "cooling off" area, a.k.a. "catch can".
Some cars (like my wife's old neon) had a bad habit of picking up oil, injecting it into the intake and burning it (or having bad seals as well), so in the air filter area there was a small filter that would get filled with oil. Installing a catch can would "catch" the oil and save the filter.
I would think that Ford would have figured this out if it really was an issue. If the engine is designed with a turbo, I would think that a catch can would be unnecessary. If you put a turbo on an engine that wasn't designed for it, you might consider putting one on.
Others can probably enlighten more than I did (the small knowledge I have of the topic).
~egon