Face, neck and other areas can lose volume due to ageing, excessive weight loss or other causes. Fat is a natural filler. All fillers that are used for facial rejuvenation, add volume to the areas where it is needed.
Fat is harvested from your own body, so the risk of allergic reactions is less. It also contains stem cells which maybe makes it superior to other fillers.
Giving a natural looking result really depends on the experience and expertise of the injector. It is delicate procedure and the injector needs to be familiar with the facial anatomy to know exactly where and how much to inject. In the right hands, both can give excellent results.