As far as i know it collects any gasses and vapours that PCV valve lets past.
These vapours are normally rerouted back into the intake for the engine to burn off but can end up clogging up the intake over time.
The oil catch can collects the oil mist and condenses the fuel vapors while allowing "cleaner" gases to be passed back into the intake.
It means you have the catch can to empty and clean out also during your regular oil change, but the stuff these things collect is quite suprising.
Some of them are even filtered on the top to allow the excess oil vapours to be released to atmosphere.
Hope this helps and im sure someone can correct me if im wrong
