What is the primary function of surface currents?

The primary function of surface current is to distribute heat from one place to another place around the globe.

Surface currents move a great distance and can also reach greater depths. They do not change with the weather because they depend on factors that do not change which includes the manner in which the Earth rotates, the shape of the ocean basin, and lastly the wind patterns.

Earth's rotation steer wind and ocean current the same way it affects free moving objects. Objects appear to follow a curvy direction while in actual fact it is moving in straight line. Global wind systems blow in one direction depending on the solar radiation caused by unequal heating of Earth by the Sun.