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.
This can be done in more than one way. Mutation in asexual system can lead to adaptation but also leads to a high chance of useless mutation. Natural selection can iron out the bugs in the system providing there is not to much mutation. See the section on Complexity for details of why this is important. The best current solution is recombination of genes and the process for this is sex.