Hi.
Every engine has to breath. The products of combustion and inevitable leaks past the piston rings would pressurise the sump and block if this small amount of pressure wasn't vented. In days gone by the rocker or cam cover would have an oil filler cap with gauze in it and the fumes would be vented to the atmosphere, some cars also has a breather on the side of the block.
Today due to regulations the breather fumes are vented back to the air intake on the throttle body side of the air filter and also a small bypass tube sucks the fumes below the actual throttle flap so the fumes will be burned on tick over.
What also happens is some of the oil from the engine as it splashes around is drawn up the breather pipe and it gets drawn in by the throttle and this collects in the inlet manifold. it will contaminate the MAP, carbon up the inlet valves and if excessive damage the Lambda sensor.
The Catch Tank literally breaks the breather pipe and oil will drip into the tank rather than it getting into the inlet manifold and engine. Both the inlet and outlet on a catch tank don't face each other for this reason, It is possible a tiny amount of oil still could be drawn in but that depends on the catch tank design.