Mitosis is cell division that results in two daughter cells with the same chromosome number as the parent cell. It is a way of manufacturing more cells for growth, or to replace old, worn out cells, for repair. Mitosis is part of the cell cycle. After interphase occurs, a time when the cell grows and synthesizes what it needs, the cell is ready to undergo mitotic division. Before it does, the DNA has been replicated. Therefore, the number of chromosomes present in the parent cell have been copied, resulting in an additional set of chromosomes. The cell passes through prophase, metaphase, anaphase and telophase. At the end of telophase, the cytoplasm divides by cytokinesis and two daughter cells are the result. They are half the size of the original cell, with the exact amount of chromosomes as the parent cell.
Mitosis ensures that the daughter cells are identical to each other and to the parent cell. Therefore, it ensures the maintenance of the chromosome set.
Hence, in this light, one of the primary functions of mitosis is growth. It allows the organism to grow and to develop by continuously dividing and producing new cells which are all identical to each other. One of the common examples you could take is in the root tip of a plant. The cells divide continuously by mitosis and hence the roots grow in length as well as the plant. The same concept is applicable for growth in human beings.
The second important function of mitosis is the repair function. This is best illustrated using an example related to human beings. When there is an open wound, the neighboring cells will divide by mitosis to repair the damaged skin. This of course occurs after the formation of the clot and it is the new cells which you can see as white smooth skin when your wound is healed.