This is a drag-and-drop target for creating a symlinked subset out of a collection of files by dragging from some other application into this window.

I use this for organizing pictures by category: open a by-date directory with some DnD-capable image gallery app or file manager, open an instance of droplinker in a directory where I'd like to create symlinks (for example my directory for bird pictures, or the directory where I keep landscape pictures for use as desktop wallpaper), then I can drag pictures into it to be symlinked. With typical file managers you have to hold down some modifier key (can you remember which one?) to create a link instead of moving or copying; but droplinker only makes links. It could be used like that to make subsets of any collection or to make a different directory structure pointing to pre-existing files: music or e-books that you want to put on your phone, documents that you want to print from another machine, etc.

You can of course open multiple instances of this program in different directories if you are mass-organizing a lot of files in this way.

Another useful trick is to then use rsync -L to copy the actual files to another machine, without making copies locally.