Why is it important for cells of multicellular organisms to undergo mitosis?

Cells of multicellular organisms (most plants & animals) go through mitosis in order to increase the number of cells in the body so that the body may develop initially, develop in later stages of life, and repair itself. The latter function, in most plants and some animals, may include regenerating (re-growing) a lost body part, such as an arm of a starfish.

Unicellular organisms (e.g. bacteria, yeast, some fungi) undergo mitosis as their means of reproduction. Instead of "having children" or "laying eggs" the single-celled organism simply divides itself into 2 single-celled organisms.

MeiOSis is undergone by male germ cells in species which reproduce sexually (rather than asexually) in order to form sperm or pollen cells and by germ cells of females of the same species in order to form egg cells. When the sperm and egg (or pollen and egg) unite then a new cell with the same whole number of chromosomes as the parents is formed and this new cell develops into an individual of the species