Most organizations simply want to know that you are genuinely interested in serving the same mission they seek to fulfill. Again, do your research on the organization and show them you support the same goals as theirs.
"I want to use my skills to serve a less fortunate population, because I am passionate about helping others and educating them about ways they can better themselves."
"I am interested in volunteering to give back to my community and build relationships with likeminded people. These are the experiences that make all the difference."