An organism can get new cells from either cell growth where the numbers of organelles increase or during mitosis.
Meiosis continually reshuffles the genes resulting in a great variety of offspring. Without meiosis we would all look exactly alike. Meiosis allows each offspring to be different, and so potentially better, than the parent. If a disease comes along, there is a good chance that some of the population won't get sick or die because everyone's DNA isn't exactly alike.
