This is an important and practical question. If we define social welfare as the well-being of people in society, especially people who are less fortunate, then social work is directly related, because social workers help those who are in need. Social workers have training in how to deal with people and they know what programs are available to help them. In a word, they know the human needs of a society and have ways in which to meet those needs. Hence, they can help in many areas: family abuse, alcoholism, teen pregnancy, and education, to name a few.
Now it is also important for social workers to be in involved in policy making on many levels. This might seem like extra work, and it is, but this step is essential for two reasons. First, they know what the people need as they spend time with them. Second, they must convey these needs to those who make decisions and budgets, so that the right resources are given to those in need. Without influence in the area of policy, there might be budget cuts where people are truly in need.