Random mutations are a key ingredient in the neoDarwinian model of evolutionary change.
They guarantee, at least in the minds of those who believe it to be true, a source of novel variability on which natural selection forces can work to drive the process of evolutionary change. Many of us have been unimpressed by the combined power of mutations and natural selection to achieve the diversity of body plans and complex organs found in the living world. We discover in organisms the kind of complexity which forces the conclusion that they have been designed and created by God. We do not expect animals, nor even a single cell, to be products of natural processes. To our minds, this is far less likely than finding a watch, or a factory, that had not been designed by man and constructed by human skill.