The added complexity and cost of a liquid cooled system gives better performance. The maintenance and component costs are increased, but a well designed liquid cooling system will cool more effectively than an air cooled one. Also, the liquid CPU cooler can operate much quieter than an air cooled unit because it can have a bigger radiator which uses a lower speed fan (or perhaps no fan at all). The coolant transfer lines will also dissipate heat.
Fundamentally, all the heat from the CPU must go into the room. An air cooled CPU relies on moving air, which requires a lot of air or large surface areas during higher heat loads. Water cooling takes advantage of the cooling capacity of water (4 times that of air) and the higher mass of water (1000 times that of air). This allows water cooling to remove more heat at the CPU by moving the parts which exchange heat from the room outside the case.
To regulate temperatures, the temperature of the coolant can be varied, the flow rate varied, or some combination depending on the system. Personally, I buy cases designed for good ventilation and large fan copper CPU heat sinks for simplicity.
Regards,
Malta