I'm forever telling students there is often merit in doing work for free. They look on in horror as the words leave my mouth, till I go on to expand on the many positives that can come from such a thing.
Here's probably the best example in the world. In 1977, William S. Doyle, Deputy Commissioner of the New York State Department of Commerce hired advertising agency Wells Rich Greene to develop a marketing campaign for New York State. Doyle hired graphic designer Milton Glaser to work on it, who in turn expected the campaign to last only a couple months and thus did the work pro bono. It became one of the most recognised and imitated logos in the known universe, copied and reprinted billions of times, and becoming completely synonymous with the city.
The Museum of Modern Art in NYC has Milton's original sketch on a bit of paper, together with the pre-Photoshop-era artwork. It's about 3" wide. Who would have known the enduring power, so many decades on, of these four crayonned characters.