On modern gasoline engines there is always a bit of blow-by as the violent explosions in the cylinders cause a bit of exhaust to squeeze past the pistons and piston rings. This puts a lot of extra pressure, unburnt gasoline vapors, etc., into the area where the oil is being used to lubricate things. Back in the stone ages this extra pressure and unpleasant gasses, plus a bit of oil and oil vapors from the crankcase, would be vented to the atmosphere. You may remember some classic cars had a small round piece on one of the valve covers that doubled as a vent. This was a source of pollution so the engineers came up with shunting the gasses, unburned hydrocarbons, and oil vapors through a hose to the positive crankcase ventilation valve, and finally back to the intake system so those hydrocarbons and vapors could get burned in the next go-round.
On fuel injected engines there are a couple of places where engineers could place the injectors. Originally the fuel was squirted in before the intake valves, but then they discovered the engines could deliver more horsepower and fuel efficiency if they could direct inject the fuel into the cylinder itself. My 2016 Ford F150 with an Ecoboost 3.5L six cylinder, twin turbo engine is of this type.
Tying both points together, when they originally injected the fuel before the intake valves, those nasty burps coming from the PCV valve would sometimes get a bit of oil or oil vapors on the intake valve itself and then get baked on as carbon. No worries though, since the next cycle for that cylinder would have the fuel injector squirt some gas and any additives the petrochemical companies added to their fuel. Gasoline is an excellent cleaner and it would clean off the carbon on the valves.
On the direct injection engines, there is no cleaning burst of gasoline on the back side of the intake valves. It turns out that the carbon can build up and cause some serious issues, sometimes even requiring the engine to be dismantled and manually carbon cleaned for a few thousand dollars.
Here's where catch cans come in to save the day. Originally a racing or high-performance modification, adding a catch can between the crankcase vent port and the PCV valve will allow those oil vapors and bits of liquid oil to go into a small canister, through a filter that drops the oils into the bottom of the removable can, and allows the unburnt gasoline and blow-by gasses to continue on as normal. Every oil change, you would unscrew the catch can, which is around the size of a soda can, and dump the collected oil.
By installing a catch can, you're intercepting the oils that cause the carbonization issues in your engine. Installing one on my truck took fifteen minutes and cost me $150. By doing this plus switching to full synthetic oil (which has less evaporation when heated), I won't have the problems that come with a carbon-clogged engine.
The nice thing is that engineers realized this was a problem and on newer vehicles switched to a dual-injection system, both direct-inject and another injector cleaning off the intake valves. The downside is that you have twice the number of injectors to replace when they eventually go bad.
If you have a direct-injection engine, I would recommend adding in a catch can.