A project is a cluster of materials united by a common subject or designed for a particular group of learners.
Projects come in handy when several professionals are working on the same material or a number of content items. Depending on the settings, a project may be available to the entire team or to some of its members.
Different teammates can be given different access levels. For example, for an illustrator, the read permission will be sufficient to look through the text of a page and come up with ideas for visuals. But, the subject-matter expert will definitely need the write permission to edit the page: correct factual errors and add some important details.