Document Sets can share metadata with their contents, making it easier to tag and manage content. Workflows can be run on individual items within a set, or on the Document Set as a whole (ex. treating the Document Set as a legal case file, where once the case is closed, it is routed with all its content to a Records Center for storage and eventual disposition).
Brought online as a feature of SharePoint 2010, the SharePoint 2013 Document Sets continue to refine the capabilities of this encapsulation method.