Why would a cell go through the process of mitosis?

Mitosis is defined as nuclear division, meaning that at the end of the four phases of mitosis, there will be two nuclei and the cell will be preparing to divide into two by cytokinesis.
In unicellular organisms, mitosis is a type of asexual reproduction. Single parent cells use mitosis to reproduce, creating two daughter cells that have no genetic variability from the parent cell. In multicellular organisms, mitosis is used for growth and repair. For example, as you grow taller, your body makes more skin cells, muscle cells, bone cells, and so on through mitosis. In this case, mitosis is used to produce new tissue so that you can grow. Also, mitosis is used for repair of tissues. For example, when you get a small cut on your hand, skin cells are made via mitosis to repair that cut.
However, it's important to remember that mitosis is not the division of the whole cell. In order for the whole cell to divide, it must undergo cytokinesis, which is the division of the cytoplasm, after mitosis.