Higher frequencies can be reached by increasing the voltage being delivered to a chip. Increasing voltage really increases the heat output though, so you need better cooling.
Water and Vapo cooling simply allows you to pump more voltage through without overheating, which in turn means higher speeds are possible.
Another good thing about water cooling is noise. because you can rig up all the components (Chipset, CPU, graphics card) into a single 'loop', you can effectively transfer all that heat away to a single large radiator which then only requires a single large low-speed fan. Because you're replacing 3 smaller noisy fans with one quiet one, you get a huge noise reduction.