Document type homepages make it easier for authors to find the right location for a new document. Typically authors should store a new document dependent on invariant information of the document. That is related to information that will never change. Thinking about what information this could be it almost always boils down to a very limited number of metadata information: creation date (and maybe user) and the type of document.
All page wizards for projectdoc blueprints provide a tool to store a new document as a child to its document type homepage instead of the current page.