Without these means there would be no sure fire way of reproducing even identical off spring - the rules of entropy put paid to this. DNA is the instructions the body's cells follow to create and run all its functions, like a look up table or hand book for each cell. A asexual creature that reproduces effectively recreated a clone of its self. This is fine up to a point and is a useful tool for creatures with a limited life span but can cause problems for more complex life forms. The problem is no room for adaptation is allowed from generation to generation.
Variations are important in the survival of a species.
A species has to survive in an environment and few environments are ever stable, the main reason being that of hunting and being hunted. In fact for a species to survive it has to take part in an evolutionary arms race. The Red Queen principle, taken from Alice in Wonderland, says that species has to change in order to survive. In essence to stay still a species has to keep on moving forward, and moving forward is done by way of adaptation. Adaptation can only come about through alteration.