Here's why:

* Diploid cells have two sets of chromosomes (one from each parent). This is true for most cells in your body, called somatic cells.

* Haploid cells have only one set of chromosomes.

* When a sperm and egg cell (both haploid) fuse during fertilization, they create a diploid zygote, which will develop into a new individual.

This process of halving the chromosome number in gametes ensures that the offspring inherits the correct number of chromosomes from each parent.