It condenses oily vapors from the crankcase PCV / Breather hose system before allowing it go back into the engine via the throttle body or intake so that only air returns and not oil. Although there is question about their effectiveness in daily drivers. It cuts down on the oil that is burned or that can leave gunky residue on things like valves etc.
If you do put one in put something in that will help the oil condense before returning because of high under hood temperatures it could keep the oil-vapor in vapor right through the entire can and back into the system. Some folks will put the can in the bumper or in front of the radiator to keep it cooler and condense the vapor. Just make sure where you put it, it is accessible because it will need emptying if it catches anything.
Most of the catch cans I've seen that are effective usually have a steel-wool stuffing or wire mesh to not only increase the surface area of the 'catch' material and thereby make it more effective, but the hidden and likely main reason for this is that since the PCV gasses have to navigate a longer path through the steel wool, the gas cools off significantly. This may be why some folks stick the can in the bumper or in the airflow- they dislike the slight mess the mesh creates and think it's unneeded, hindering the performance of their can.
P.S.- 'most' port-injected daily drivers will probably not 'need' a catch can. If you have a GDI engine, it may certainly help with both LSPI and intake valve deposits.
Something else to consider with your direct injection engine is the volatility of the oil. Less evaporative loss is a good thing but with DI engines it means less to gunk up the intake and valve stems. The Petroleum Quality Institute of America has lists of PCMO tests where you can look at properties of many different brands pretty easily: http://pqiamerica.com/PCMO_Sample_Summary_12_15_2016.html
You don't need a catch-can or worry about oil volatility...but this is BITOG.. every incremental little bit is mandatory.