Yes, charities have a core of paid workers. Volunteers come and go, and take their abilities with them, but permanent workers provide the continuity and specific skills. There are many roles which can only be done by those qualified and experienced in certain subjects.
The charity I volunteered with employed administrators and chartered physiotherapists, for example, but there were also volunteers who gave their experience part-time with other therapies, or with unskilled tasks, and students getting practical experience by volunteering in their free time. Relying on the volunteers totally would not have given the continuity of service and treatment needed.