Your PV/Wind turbine system sends most of its electricity back onto the grid, especially if you're out or you don't consume much energy. Why export your generated energy? you could use it to heat your water (if you have an immersion tank) or use it for space heating, reducing your hot water/ space heating bill by pre-heating during the day with the free energy from the sun.. with a power diverter, you are in control of your generated electricity..
This is a complete system overview by Myenergi (as an example)