Two years ago you couldn't have paid me to give my writing away. Seriously, even the idea of getting my name in print did not over shadow the need for compensation. Today I believe there is a time and a place for everything and that does include giving away your writing.
1. Compensation does not have to be green. There are times when you will be able to write for someone and get compensation more valuable than money (if that's really possible). Getting a link to your blog or recognition from your peers can be almost as valuable when you are trying to build your expertise.
2. Writing for the sake of writing can be therapeutic for you and for your reader. It can also lead to other opportunities along the way. Having an opportunity to share an experience or an answered prayer (or just some answers that we all need) or being able to work through the situation on paper can be all that you need for writing that piece (or peace - depending on how you look at it).
3. Reaching out to help others IS the best way to write for free. When a friend or a colleague needs a post for a website, when a new writer needs some help with a story or when a student needs some guidance from a mentor - each of these opportunities to write for free will offer more benefits then even the best pay check.
4. Building a business requires a LOT of work without compensation and writing is not different (particularly if you are creating an online business with your writing). Each word that you write has value that will come back to you down the road.
5. A good cause is always a reason to donate resources and if your resources happen to be your words then so be it. You may not get the nod from Uncle Sam for your donation but you will be blessed by your giving.
Writing for free is not always about writing for free. Sometimes you are sharing your gifts and talents because others shared their gifts and talents to help get you where you are. Other times you may just be writing for your own sanity. No matter what there reason is there are times when you may want to write without getting a paycheck in return.