Are catch cans necessary? For some engines, it is a 100% engineering necessity for engine operation. Diesel engines that ingest their own crankcase gases, for one. Ask anyone who has ever worked on a diesel engine that had a failure of said system due to neglect. Awful mess, and exhaust smokes like a demon. If it's an EGR engine, it is also likely to completely occlude its grid heater (if it has one) with a pumice-like caking that is almost impossible to remove (treated by replacing grid heater completely).
Are catch cans necessary for gasoline engines? For some, yes. That's why they come with them from the factory. Turbocharged models in particular. German engines in particular. When the oil separators fail, they become complete cropdusters.
Are catch cans necessary on engines that never came with them? Age plays a larger role here. As blowby increases, so does an engine's tendency to put the engine oil straight into the PCV system. Witnessed plenty of engines capable of evacuating the entire contents of the crankcase in a single day. We have a gasoline NPR that used to do that. Does since rearranging the PCV system to draw out of an extended oil filler modified into a catch can. Other engines drive off of the edge of the world without ever having had one.
The larger role is the effectiveness of the design. Some catch cans are professionally engineered to do their job correctly. Some are just a bunch of over-glorified soup cans that really don't do anything at all, and cost a ton.
A proper catch can could likely help an engine from loading up with deposits, but few are far between are actually that.