When your batteries are full, you need to divert the excess power being generated to a separate load so your wind turbines will not go into high speed freewheeling in heavy winds. That is where a dump load (also called a divert load) comes in. The charge controller will switch from battery charging to sending power to the dump load to keep the wind turbine under load.
Not using a reliable dump load, or no divert load at all, can cause serious heat build up in the wind turbine motor which could ultimately cause the motor to overheat, seize up, and burn out the stator. At Missouri Wind and Solar, we use wire wound resistors in our charge controllers to make sure all that power has a failsafe place to go.