I can hear the exasperated groans of people already. If I had a nickel for every time someone told me not to give my work away for free, I'd probably have a few dollars. "But you're killing the photography industry!" "Taking business from REAL pros!" "Devaluing all of our work!"
As a freelancer, you really have (in my opinion) no reason not to volunteer. Here are just a few reasons why I give away my time and work to causes that I believe in, whether or not they have an advertising or photography budget or whatever budget that could pay a photographer to work for them.
1. Networking. Fact is, it's an easy way to meet people and get the word out. The more people you talk with, the more people will remember "that photographer kid who volunteers at ____" or "that graphic designer guy who I volunteer with" or "that girl who is a freelance writer who I see when I volunteer." This gets you into their head. Talk it up, meet people, they'll remember you.
2. Referrals. I've gotten probably more than my fair share of referrals from people that I've met via volunteering. Whether it's referrals for my architectural/real estate work, or referrals taking pictures of people's kids, or pictures of pets (which admittedly, are not my specialties), it doesn't hurt one bit. People are going to be more inclined to work with someone who they know takes an interest in their community or a cause that they align themselves with, and thus, will go to you when they need your services, rather than some random guy who doesn't take an interest in local causes or is just relying on an outdated business model to find work. Get out there, be a good human being, and work will follow.
3. Portfolio building. I have a ton of respect for the big name photographers. Here's another spot where I often hear "never work for free!" but the truth is, I'm pretty sure that Chase Jarvis, John Harrington, Zack Arias, David Hobby or [insert big name photographer here] didn't hang out a shingle and start invoicing $2500+ per shoot the next day without a solid portfolio built over months, or more likely, years. Volunteering is a low-risk way to build a photography/design/artistic/whatever portfolio in a low-pressure environment, without pissing off pros who are trying to make a living in a big-budget field where free/cheap work would be frowned upon.
4. You're a damn freelancer. I'm assuming that anyone who is a freelancer did so because they were sick of getting up at 7AM, spending all day in a cube, having basically zero freedom in terms of how you use your daylight hours, you wanted to use a talent of yours for money, etc etc. Whatever the reason, being a freelancer, you are flexible, probably have more free time than most (though sometimes I wish I didn't, because I'd prefer to be working than sitting around, but still, you know what I'm getting at). So take some of that extra sitting-around time and go do something cool with it, even if you aren't getting paid. You have talents that will be appreciated and will help out a cause that you might believe in.
5. It feels good. Really the most simple answer here, and should probably be the first point in my little bullet list, but then nobody would read the rest of the post. It should be something you want to do not for financial gain, that would be getting into it for all the wrong reasons. Do it because you want to, be enthusiastic about it, and you just may experience some nice side effects that will help your freelancing business along.
I personally volunteer at a local humane society, as you might have known if you have poked around my galleries at all. I take cute (ideally) photos of their animals for the website, with the intent of stirring up interest to would-be adoptive parents. I didn't get into it with the intention of making any money from it, as I had no idea how animals and architecture were connected, or if anything would even happen, or if they would even need my help - I just like to take photos, and I like dogs. So the solution was obvious! But regardless, it's been very rewarding and I've gotten more than I thought possible out of it. So try it out. Pick something you like, I'm sure there's a local non-profit or similar that would love your services and be glad to use your talents, especially if you're a freelancer in a creative field.
Here are a few shots from my work there.