For EJ type engines they have a cooling problem on cylinder 4 from factory. A kit includes a coolant hose and a Hose Tee allowing you to split the coolant line going into the heater core to cylinder 4 coolant exhaust port letting coolant flow more efficiently and keeping cylinder 4 cooler.
The pro is rerouting your coolant line keeps the cylinder cooler preventing serious damage to the motor when you are driving the car hard. The Con is your radiator fan will kick on more often due to the coolant pulling more heat out of the engine causing coolant temperature to be higher.