Why must a gamete have only one half the number of chromosomes found in body cells?

We are diploid organisms thus require two sets of chromosomes. In order to ensure the zygote is a diploid (has 2 sets of chromosomes) the gametes must be haploids (has 1 set of chromosomes) as two gametes join to make a zygote.

Having half the set of chromosomes also ensures that the offspring receives half of its chromosomes from its father and half from its mother. This becomes very important as to ensure there is genetic diversity in the offspring, both the fathers and mothers genes are needed.