In order for the zygote, the fertilized egg cell to have a diploid number of chromosomes (2N) the gametes must have half of that number (N).
Chromosome pairs are divided into single chromosomes in gametes in order to be fused with another gamete with single chromosomes as well.
This way, the zygote gets one set of chromosomes from its father's side and one set of chromosomes from the mother's side. The zygote uses this information gained from both of the parents to develop into an embryo and later into an adult organism.