Why is a balanced frequency of different alleles of a gene in a population more useful for the survival of that population facing environmental changes?

For the trait conditioned by two alleles, for instance, A and a, a balanced frequency between the alleles A and b is more advantageous for survival. For instance, in an environmental situation in which the aa homozygous phenotype becomes incompatible with life the presence of a good number of individuals AA and Aa will result in better survival chance for the species. Another instance: an environmental situation in which dominant phenotype becomes incompatible with life; in this case the existence of heterozygous and recessive homozygous individuals in enough number may be fundamental for the survival of the species.