Most actors do free work because they’ll Get Exposure, Gain Experience, or in some undefined way Get Ahead. At least, that’s why we think we work for free.
The truth of the matter is that most of us perform for free because someone liked us enough to ask us to perform.
Part of being an actor is dealing with constant rejection (submissions for auditions, the audition itself, the callback, the lack of a callback, the reviews, the applause, etc). We put ourselves out there a lot, and most of the time, we get nothing for our efforts. So when someone asks us to work–even for free–we get a sense of reward and accomplishment, and we didn’t even have to do anything yet!
And that feeling is nice!
But a feeling is not enough reason to work “for free”.