Why&How is being heterozygous for the disease beneficial to people living in west central Africa?

If you mean hetrozygous for sickle cell, this trait is useful in areas where malaria is endemic.  The hetrozygous form of the disease causes slight twisting in low-oxygen conditions and this twisting is sufficient to crush the malarial parasite in infected cells and prevent them from living and reproducing.  Please note that two copies of the gene are fatal.

Certainly not all mutations are bad.  The majority of them are probably neutral and a very small number are beneficial.  Some genetic disease persists because although they shorten life, they do not prevent the carrier from reproducing.

One example of this is Huntington's Chorea, which destroys the nervous system in later life.  Until recently, there was no way for anyone to know whether he or she was afflicted with this gene and since the person would have already had children before the disease emerged, no way to stop it.