Describe wind farms. why are they useful?

They are used to generate electricity , wind power is clean, 

A wind farm is turbines in a same location that produces electricity, is wind turbines that cover a big area of hundreds of square miles and they are used for a lot of purpose like for agriculture or electricity.

Ghana, first of the great medieval trading empires of western africa (fl. 7th–13th century). it was situated between the sahara and the headwaters of the sénégal and niger rivers, in an area that now comprises southeastern mauritania and part of mali.