The idea is simple as well as counterintuitive: if you want to grant public and free access to a work of authorship, it is best if you establish that such work is protected under copyright law. In this way, you can require that further publication and derivative works may be subject to certain conditions: e.g., to be licensed under the same license, for free, with no limitation for derivative works and with credit to the original author.
Otherwise, if your creative work were to fall in the public domain, anybody could claim a proprietary right on derivative works.
In a recent case decided by the Court of Appeal of the Federal Circuit (Jacobsen v. Katzer and Kamind Associate, Inc.), the Court addressed - among the others - an interesting issue, namely whether it is possible to enforce an open license, given the (alleged) lack of economic interest by the copyright holder.
Eventually, the Court acknowledged that the conditions i) to give credit to the author and ii) to display the copyright notice, represent a sufficient economic interest to uphold a public license. In fact, giving credit to the author -in the case of an open license- is not merely a moral right, it rather encompasses an inherent benefit for the author.
In particular, the Court defined such benefit as follows:
"The attribution and modification transparency requirements directly serve to drive traffic to the open source incubation page and to inform downstream users of the project, which is a significant economic goal of the copyright holder that the law will enforce. Through this controlled spread of information, the copyright holder gains creative collaborators to the open source project; by requiring that changes made by downstream users be visible to the copyright holder and others, the copyright holder learns about the uses for his software and gains others' knowledge that can be used to advance future software releases."I am not absolutely sure that such statement is truly catching the spirit of the Open Source project. However, it looks like a robust argument and a preeminent endorsement of open licenses. Somehow a windfall.