Whatever you write, there are two things that I would make sure to include.
1) Social Work or helping people has always felt like a calling. Support with experiences, thoughts, and feelings.
2) NASW ethics and standards. Read up on them, and show them that you embody those standards via experiences.
Also, be aware of a particular school's mission statements and the kinds of research the faculty does. For example, you don't want to say that you want to go to X school because you want a good foundation in family counseling when the faculty research focus is communities, policies, and organizations.
Graduate schools want to give their students the best experience they can. In order to do that they need to know that they can meet your needs and expectations. Be sure to drive home that you're a fit for a school and why.
