Licenses are legal documents which specify what can, or cannot, be done with a copyrighted work. For fully restricted, copyrighted materials, there is generally no license. Rather, copyright holders rely on the legal restrictions placed by federal laws.
When we speak of 'open' licenses, however, we generally speak about the instances in which an author decides they do not want the "All Rights Reserved" standard copyright restriction, but rather have decided to instead only reserve some of the legal rights to the work. Thus, open licenses generally grant users expanded permissions to distribute and use a work - permissions that are not normally granted by copyright law.
The benefit of these licenses for the authors is that they can be assured that their legal rights to their work are protected, while simultaneously ensuring that their work will reach a much wider audience. By making their works openly available, they have the greatest opportunity to reach users, in turn increasing the dissemination and proliferation of their contributions.
Additionally, open licenses give users the confidence to use resources in new and innovative ways, knowing that they have express, legal permission to use the work according to the terms provided by the license.