It works the same as an oil seperator, in that they prevent the engine crankcase fumes from entering the air intake, and potentionally fouling the throttle body plate.
I had a homemade catch can, and from personal experience, saw the oil collected. Not a huge amount, but enough that had me convinced that I was doing better to keep as much away then to do nothing at all.
If you have a tired engine, then perhaps the use of the small filters will allow some of that oil to get onto the COP/head covers (formally known as valve covers).
In any event, you only need a catch can or filter on the passenger side of the engine, as this is the side that the crankcase fumes are drawn from and routed to the intake. On the drivers side, fresh air is drawn in from the air cleaner box/tube into the engine via a tube that connects to the head/valve cover. The crankcase draws in this air, and then the crankcase fumes are expelled (via the PCV) on the passenger side to the intake.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.