Why is it necessary for each gemete to contain only half as many chromosomes as a body cell

This is for the offspring to show traits that come from both the mother and father.

Explanation:

Gametes are formed by a kind of cell division called meiosis in which the chromosome number is halved. Thus the said gamete cell is said to contain a haploid number of chromosomes.

The fusion of two haploid gametes is called fertilization. It results in the formation of single celled zygote which has a diploid number of chromosomes. The zygote now divides and develops to produce the offsprings whose somatic cells are diploid.