Often, you may want a number of related documents to be treated as one entity, corresponding to a work product in your organization’s business process. For example, an expense report submitted by an employee might include a number of bills/receipts, travel tickets or boarding passes, and other supporting documents. Document sets allow all these documents to be grouped together thus facilitating the entire set of documents to participate in the business process as a single entity.
In SharePoint, document sets are implemented as content type. Hence the traditional concepts applicable to a content type (workflows, versioning, and so on) apply to a document set as well.