The interviewer wants to know what drives you to help others. This question is a further test, by the interviewer, to convince themselves that you have a passion behind your volunteering ambitions.
Interviewers are trying to avoid picking candidates from the crowd of people who say, 'Well my Mom told me I should help volunteer...' or 'I heard this is a great place to make friends.' Sure making friends is great, but most would instead choose a candidate that volunteers because they are passionate about what the organization provides to the community.
Entry Level answer example
"I love helping others, especially the elderly because there is a great joy in being a servant for those in need. Your organization helps the people who I care most about, and I would love to be a part of it."
Experienced answer example
"Helping others is my innate nature. It is part of the fabric of who I am. I think it has a lot to do with how I was raised. My mother always volunteered her time to local causes and feeding the homeless. I was brought up with this value at the forefront of living, and it has been a rewarding part of my life ever since!"