Social work is one of those fields where there a lot of people who have regular paid jobs but there are also a large number of volunteers. In some cases people volunteer because they are generally interested in helping people. However in most cases people are using the volunteer work as a way to get their foot in the door. Regardless of the reason there are plenty opportunities to work as a volunteer.
While most people who work in the social work field would prefer to be paid for their efforts there are lots of people who volunteer as well. In most cases the reason that they volunteer is to use it as a stepping stone to a paid job. If nothing else volunteer experience looks great on a resume, especially if it is in field that is closely related to the one that you are interested in working in. This is especially true in the world of social work where employers are generally looking for people who genuinely interested in helping people. Nothing demonstrates that better than volunteer work.
In a lot of cases volunteer work can lead directly to a job with the same organization that you are already working with. Hiring volunteers to take on jobs as full time employees works well for both parties since it gives the employer a chance to see what you can do and it gives you a chance to see what the job entails. It also helps that you are going into the job with some experience and a knowledge of what you are doing.
Many times the volunteer work isn't really volunteer work so much as it is a requirement to get the qualifications that you need. In most cases when you enroll in a degree program in social work you will need to complete a minimum number of hours as a volunteer. This is in part to give you a chance to make sure that social work is a good career choice for you. It is also to make sure that you have some hands on experience when you graduate. Social work isn't really the sort of thing that you can learn just in the classroom, it takes some real world experience.
One thing to keep in mind if you are considering taking on a volunteer position as a social work is that you generally can't just show up and start working. In many cases the requirements to be a volunteer will be the same as those for a paid employee. That means that you will need to meet educational standards, you will need a background check and you will likely need a number of vaccinations as well. In most cases these are legal requirements so there is no getting around them. The standards may be lower for volunteers, especially with regards to education but they will not accept just anybody.