By volunteering in your community, not only will you be helping out organizations in need, but you'll also expand your network, add something to your resume, and come in contact with loads of new people who will now want to help you.
And, of course, if you happen to want to work in the nonprofit sector, volunteering for a nonprofit that particularly interests you is a great way to get a foot in the door. You'll get to meet inside players and form relationships, get early leads on upcoming openings, and be able to demonstrate that you're reliable, talented, organized, efficient, skilled, and all the other things people look for in new hires.
Here's the most important part: By volunteering, you become a known quantity. If I have a candidate who's qualified for a job and she's a known quantity--meaning that I know from direct experience with her that she's reliable, competent, sane, etc.--I will almost always go with the known quantity over a marginally more qualified candidate who is a stranger to me.
The reason for this is that you simply can never get to know someone as well in interviews as you can by actually working with them. The candidate who seems great in interviews can end up being flaky, disorganized, difficult to work with, all sorts of problematic things that someone can manage to hide during the hiring process. But someone you've actually worked with? You know what you're getting. And volunteering lets you become that known quantity.
(Of course, you have to be a good known quantity. That means you should treat your volunteer work as seriously as you would a paying job.)
The worst case scenario is that you don't end up being offered a paying job there--but you've spent time helping a charity you feel good about, you've made new contacts, and you have additional work to put on your resume. As worst case scenarios go, that's a pretty good one.
Alison Green is the author of Managing to Change the World: The Nonprofit Leader's Guide to Getting Results. She is chief of staff for the Marijuana Policy Project, a nonprofit lobbying organization, where she oversees day-to-day management of the staff as well as hiring, firing, and staff development. Her writings have been published in the Washington Post, the New York Times, Maxim, and dozens of other newspapers. She blogs at Ask a Manager.