As I understand the term, “community welfare work” is used in Australia to mean much the same thing that “social work” means in the United States. Community welfare work is linked to sociology, then, because many of the problems that community welfare work seeks to ameliorate are problems that have their roots in sociological conditions.
Community welfare workers are people who use their skills and knowledge to help others who are having problems. They seek to change the social environment in which these people live so as to help them with those problems. In other words, they are not psychiatrists or therapists who are working with the client’s own mind alone. Instead, they are trying to alter the context in which the person lives.
This is where sociology comes in. In order to help a person by altering their social environment, it is necessary to understand how that social environment contributes to their problems. For example, if a community welfare worker is trying to help someone who is poor, it is important to understand what factors keep that person in poverty. Sociology is the study of the ways in which aspects of the society as a whole affect the life chances of people in various groups within that society. This means that sociology studies the sorts of factors that cause people to become and remain poor.
Community welfare work is linked to sociology, then, because sociology studies the sorts of problems that community welfare workers are trying to help solve.