Or because it could result in more sales, if they do a job the customer likes? I would not do a big job for free, but I have often done small "sample"-type work, including logos and quick and dirty letterheads, for new businesses, in the hopes of more work when they grow to need it. (My design work was on paper, back in the 1990s, I haven't done any graphics design on computer, I'm not set up for it.)