Why is the heterozygous genotype for hemoglobin advantageous in some regions of the world? What would this adv?

The first heterozygous genotype advantage I can think of is the gene for Sickle Cell anemia, which alters the shape of hemoglobin.

The double recessive alleles together code for the full-fledged Sickle-Cell anemia, which makes the hemoglobin unable to hold oxygen as efficently, and could lead to death.

If there is the heterozygous genotype, then it helps prevent malaria, because malaria holds best on normal hemoglobin.

This is why sickle-cell anemia and malaria are more common in the same parts of the world. They develope together. As malaria is more common, those with the heterozygous trait have an advantage, live longer, and spread it to their children. The trait then becomes more common in the following generation.

A variety of mutations the hemoglobin or structural proteins in the red cell wall decrease the stability of the red cell and wshorten its life span in circulation.  Malarial trophozoites develop in red cells.  If the cells break before the trophozoite matures, malaria will be a far less damaging disease.