Well one reason is if our cell is infected by some type of pathogen, if we have an infected cell we don't want that infected cell to infect other healthy cells. And so we want to kill off that cell. And this is the process that helps us basically kill off that cell. Also when the embryo is developing sometimes we actually want to kill off certain cells in a certain area. For instance when we develop the fingers and the toes to actually develop these digits we have to destroy the cells in between these areas to basically go from this to this. And this process is a result of epitosis.