Variation is essential
Without differences between the individuals in a population, populations cannot change over time—that is, they cannot evolve. Some features must exhibit variability so that change is possible. This variation arises when novel genes are produced as mutations. In other words, mutations—random genetic changes—are the ultimate source of variation in changing populations.
When a novel mutation arises, the population has changed; thus, mutation alone is a mechanism of evolution. However, the occurrence of a single random mutation is unlikely to have a profound effect on a population. Other evolutionary mechanisms—such as gene flow, drift, and selection—play a larger role in shaping the diversity we see on earth today.
However, without this pre-existing and recurring variation, we would not have an evolutionary story to tell.