Humans and other multicellular organisms need mitosis to regenerate cells for both growth and repair. Mitosis is the process of cell division. Along with meiosis, it is one of the key parts to the creation of offspring, wound healing and growth in adolescent children.
In order for a person's body to maintain itself, each cell has to have a copy of the bodies DNA. Every single cell in the body has that built into its nucleus. These cells split and divide, creating a new copy of the DNA in each cell that it creates during the cellular division process. Each daughter cell then carries an identical copy of the original DNA in it's core by the end of the mitosis process. The body's ability to heal itself when it is cut or otherwise wounded is imperative to keeping the species alive. Unhealed open wounds leave someone open for infection and even death depending on the severity of the wound. If the human body could not heal itself, then the chances and rates of death would increase greatly. The lack of cell division would also mean that reproduction itself would stop. Once the egg of a female is fertilized DNA is being passed, divided and replicated. Without mitosis, this will not happen, halting the production of the human race.