Mitosis, the process of cell division, is most important for an organism's what?

Cell division serves as a means of reproduction in unicellular organisms through binary fission. In multicellular organisms, cell division aids in the formation of gametes, which are cells that combine with others to form sexually produced offspring. For growth to occur in living organisms, the number of cells have to increase through cell division until it reaches its maximum size. 

The human body also repairs injuries by means of cell division. When skin is cut, the skin cells on either side of the cut divide to make new cells that heal the wound.