The coolant is used to transfer heat away from the CPU, and sometimes the GPU, to a larger radiator that is bigger and thus more effective than a normal tower cooler that would sit on top of the CPU. A normal tower cooler’s size is limited by the width of the case, the weight of the cooler pulling on the motherboard, and access to other components, while the radiator of a liquid cooler can sit flat at the side of the case, also enabling it to push warm air directly out of the case, whereas a normal tower cooler just pushes the warm air into the case and has to rely on case fans to push the warm air outside.