Mitosis is a process that ensures the original mother cell divides to create two cells with identical genotypes. Gaining or losing even a single chromosome can be devastating.
The figure above from Chapter 7 shows the steps that allow for high fidelity of DNA transfer. Lining up all the chromosomes along the metaphase plate positions them so that when sister chromatids separate, they end up in different nuclei. A diploid cell begets diploid offspring; a haploid cell begets haploid offspring.