Mitosis is responsible for growth, repair and maintenance of an organism because it produces daughter cells that are genetically identical to each other and thus makes available new cells specific to each tissue for growth and repair.
Damaged skin, for example, constantly repairs itself through the production of new skin cells or scar tissue. Some organisms can even use mitosis to regenerate entire body parts. Not all cells undergo mitosis at the same rates. In humans, brain cells are much more difficult to regenerate than most of the other cells such as cheek cells which are constantly being replaced.