There are so many workflow solution that exist on planet, however what that differentiates and makes SharePoint stand out as workflow solution is its capability to document centric workflows and you can even use them for document sets. Normally in other solutions you have option of attachments. Contrary to that in SharePoint, content is the first class citizen. Consider a workflow where a set of documents move with the workflow with versioning of documents feature still there, and grouping of documents also possible. For e.g we developed a loan management system for one of the leading bank of the region with loan documents being centric to the workflow. That turned into a big success.
SharePoint is a content management system. Managing Content and documents are the core of this software. One such concept that it facilitates is grouping set of documents into a set. This is enabled through a feature called Document Set.
Or you may say, Document Set is a special feature of SharePoint that can help group multiple document to make up a set and treated a single object or simply stating a "folder". e.g. as below,