The growing body of Open Access content available online (sometimes called the digital commons) provides you with a library of material that you can build upon in your own work. All Creative Commons licenses will allow you to copy and distribute the work as long as you attribute the original author, most will allow you to adapt the work to your purposes, and some will even allow you to use the work commercially.
By releasing your work under a Creative Commons license, you can share your work with the community while being assured that it is only being used in the ways that you approve. For example, you might want to allow web designers to use your photos, but only for noncommercial purposes, or you might want to allow other musicians to remix your song, but only if they share the new song under the same license.
If you want to reach the widest audience of readers, why limit it to those who can pay?
Open Access works are read and downloaded more often, and there is even some evidence that they have higher rates of citation.
Your work can become an Open Educational Resource. Open Educational Resources save students money, and a collaborative approach to scholarship and pedagogy can advance social justice by identifying and amplifying voices left out of the traditional discourse.