This allows using a shortcut from the file browser to edit the directory path. The shortcut Ctrl + L is quite standard and used in multiple GNU/Linux desktop desktop environments, Windows, as well as most web browsers. Safari on macOS uses Cmd + L.
This allows workflows such as copying a path from the command line or file explorer, then pasting it into the field without having to select it using the mouse.
The reverse can also be practical: running Save As, copying the address to the blend file, then escaping and pasting it into a terminal or file explorer, as in the following video:
The new operator is simply a duplication of FILE_OT_start_filter, I don’t know if there is a better way to do that.