GP is talking about drag-and-drop of a folder into the file picker view, which results in the file picker navigating to that folder. Or drag-and-drop a file into the file picker view, which results in the file picker navigating to the enclosing folder and then selecting the file being dragged.
This is even more useful when saving files than opening files.
I see. I have to say, though, as described, this sounds as a rather inconsistent behavior to me. Does the macOS file picker not provide file management functionality at all (e.g. creating folders or moving them around)? If it does, it's kinda strange that it would behave differently from the standard file manager.
This is even more useful when saving files than opening files.