What is the significance of balanced polymorphism?

"Balanced polymorphism is a situation in which two
different versions of a gene are maintained in a population of organisms because
individuals carrying both versions are better able to survive than those who have two
copies of either version alone. The evolutionary process that maintains the two versions
over time is called balancing selection."

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"Most plants and animals have at least two
copies of each chromosome, one inherited from each parent. The copies of the genes found
on these homologous chromosomes may be identical or different; that is, the organism may
carry two copies of one allele, or one each of two different alleles. In the first case,
the organism is called homozygous for that gene, and, in the second, it is called
heterozygous."

The
significance of balanced polymorphism can be seen with sickle cell anemia.  People who
carry two copies of the sickle-cell allele suffer horribly and without medical
intervention will die young.  People who carry one copy of of the  sickle-cell allele
and one wild-type allele develop an immunity to malaria.  The two allele balance each
other off and create a benefit for the person.