It helps you decide if the work is an appropriate fit for you, and it provides a set of standards or expectations when you do volunteer. You don't want to set aside time to work at an agency only to arrive and find there's nothing to do. Ask about training and supervision. Volunteering is an opportunity for personal growth. Don't be reluctant to request an evaluation of the work that you perform or a letter of recommendation, should you decide to list your volunteer service on a school or job application.
Enthusiasm is key.
If you are not excited about the program, the chances are that you are not going to be thrilled with giving up time in your day to go there. You get out of your volunteer work what you put into it. A young man was walking along a beach one morning, when he saw an old man picking up something and throwing it out into the ocean. Curious, the young man asked the old man what he was doing. I'm saving them, by throwing them back in the sea, so they won't die.