Basic premise, crankcase pressure vents back into the intake, which carries gunk and oil. Higher pressure on turbo'd engines pushes more gunk. The gunk collects on the intake valves. Back in the day, port injection helped clean the intake valves. Newer direct injection is downstream, which doesn't clean the valves, so they build-up and have issues. The catch can collects oil in the vent line between the crankcase and the intake to help keep the valves clean. Back in the oven older days, the crankcase was just vented, but this is an emissions issue now.
I'm sure I'm oversimplifying or missing details, but I know a lot of folks have them on this board.