There is ongoing debate over whether it is appropriate to mix business with one's personal interests (e.g., is it okay to "friend" your referral sources on Facebook?). But there's no question that your involvement in non-profit endeavors benefits your personal brand, your network, and yourself.
Your Brand: Charity involvement differentiates you. There are countless professionals whose titles and credentials mirror your own, but your community involvement is unique to you, and in many cases, more likely to be remembered by those who resonate with your cause. So, the next time your charity of choice has an event, invite your professional network.
Your Network: The next professional mixer you go to will largely be attended by fellow service professionals. But charity events attract people with either disposable time or income (or both) who have an interest in the cause. You're much more likely to find prospective clients in that mix.
Yourself: Helping others is fulfilling. Not to suggest that your desk job is bereft of meaning, but there is no substitute for an act of selfless service. It balances out the "what's in it for me?" positioning that too often permeates our business transactions. More importantly, it reminds us that, regardless of our personal or professional challenges, we have much to be grateful for.