As a full-time creative, I tend to work a lot for free. Working for free, or asking people to work for free, is generally frowned upon but for better or worse, it's a fact of life for the artistic and ambitious. Here's when I work for free.
1. It's part of the process for selling work. Probably the number one reason why I have worked for free in my career. Debut novelists must write a book before selling it. New TV writers must write sample scripts before being hired. Copywriters pitch on jobs, and so on. These are the facts of the industry and there are no shortcuts.
2. You're helping out colleagues who work for free for you. Yesterday I read another writers' pitch and talked through a pilot script idea, because she had done the same for me. Giving notes and feedback is work, but I will always do it for writers who do it for me.
3. It's fun. I've worked on many many projects for free, for fun! Fun (or "for a good cause") is a good enough reason to do anything. But it is also not an essential reason to work for free, and creatives do have to think about whether working on the project is purely pleasurable or noble, or a distraction from more difficult, but ultimately more rewarding, work.
4. There are other tangible benefits. Money is important but it is not the sole reason to work. Doing a little work for free to receive legitimate benefit such as a relationship with someone you want to know, getting experience you wouldn't otherwise get, real exposure, swag you actually want (etc), can be worth it.
Here's when to really think about working for free.
1. You're entering a competition. In the past I used to apply to a lot of writers' residences, which are a form of competition. I once spent months of work and thousands of dollars making an entry for an MTV competition I was certain I was going to win. The reality is, most of the time, you won't win, and you have wasted your time. Competitions are alluring: they seem to offer a shortcut to money, power, access and opportunity. But like I said, there are no shortcuts, and often, these are a distraction and a waste of energy. Calculate the time needed to complete an entry. Anything more than an hour or two should be considered carefully.
2. "Exposure". Exposure on, say, a national stage or at a huge conference, could be good exposure. But "exposure" is a very tricky, intangible benefit and may not be worth the time. Get really clear on what you hope this will offer and be as transparent as you can with the person in charge about it. If you're too scared to ask if it's actually going to lead anywhere, this is not a supportive environment and probably won't lead anywhere. Beware of vagary and false hope (especially your own). Do your own research. If you're being asked to, say, write for a website for free for exposure, research their traffic and make sure it's win/win.
3. Other people are being paid, and you're not. If no one is being paid for an opportunity, then the choice is yours. If some people (at or around your experience level) are, and you are not, do not work. Request payment.
4. You're doing someone a favor. If one day you can confidently call that favor back, sure. But helping someone out for something that isn't joyful or offering a real tangible benefit must be considered. Your time might be better spent helping yourself out. It can be easy to get caught up in someone else's passion and focus. But never lose sight of your own ambition. Your time and energy is very valuable. Think carefully before giving it away.