The usefulness of sociobiology has to do with its understanding of the concept that our social behavior may be affected by our evolutionary past. This is an important insight into human behavior. This may seem much more scientific than regular sociology, but I would argue that this is not so.
Sociobiology is a discipline in which scholars try to determine how our behaviors are connected to the way in which we evolved. We human beings are physical organisms and, as physical organisms, we evolved from ancestors. Over the course of our biological history, we were subjected to pressures of natural selection. Natural selection would have chosen those behaviors that helped us to survive and would have chosen against those behaviors that do not help us survive. This would mean that we are preprogrammed, in a sense, to act in certain ways. For example, it could be that we have an innate tendency to violence because our ancestors needed to be aggressive to protect their territory or their mates.
Sociobiology is useful to sociologists because it suggests that we need to think about our evolutionary past. It seems clear that evolution would have had an impact on the way in which we behave. This being so, it is important for us to try to look for these impacts. We might not be able to properly understand our behaviors (and our capacity to change those behaviors) if we do not take evolutionary pressures into account.
While this may sound very scientific, I would assert that it is not. After all, we cannot go back and rerun evolution. We cannot actually know what evolutionary pressures acted on our ancestors and we cannot know how various behaviors would have affected our evolutionary chances. What this means is that we do not really know how evolution affected our behaviors. Instead, we have to speculate. Once we start to speculate, we are really no better off than we are when, for example, structural functionalists speculate about the role of a given institution in our society. In other words, sociobiology is no more scientific than other theoretical approaches to sociology.