The earths surface is heated by the Sun's rays which are directed at the equator 0° latitude. If there were sea currents the closer you got to the equator the hotter it will get and the further away you go from the equator the colder it will get. Thus ocean currents distribute the heat at the equator around the earths oceans to even out the difference in the heat zones, so that the equator is not a desert and the rest of the earth is an ice age.