In macOS, items may be dragged onto the sidebar area of the Finder, but these do not behave as placeholders and cannot be manipulated in the below manner.
In moving the placeholder off the shelf, the actual action occurs.
The process is similar to the Microsoft Windows functionality of copying or cutting file system objects (a file or files, a folder or folders, or a combination of both) to the clipboard; the objects are not copied or removed from their original location until the paste operation to the new location is completed.
The Shelf concept, though older, is more powerful in that the file system objects, their sources and destinations are persistent and available as long as they are on the Shelf (in the Windows cut, copy, and paste metaphor the objects and locations persist until one copy/move operation is complete or until something else is placed in the clipboard).
Users simply opened a new browser rooted in a particular subdirectory, and that browser window would show the corresponding shelf, allowing users to have many different shelves based on whatever folder hierarchy they happened to be using to organize their files.