--download-only switch needs to be accompanied by another command, be it
upgrade or whatever command would require to download package. If you don't need to download packages, is plain as day that it wouldn't download anything.
--download-only restricts apt actions to just downloading the packages that it needs into the cache directory, nothing more. If your operation would end into downloading dependencies it will download them for you.
Since it needs a command like
upgrade it normally would require
sudo or root permissions, since the default directory used to download the packages is owned by root.
download command is standalone, it reads the package list and download only the package specified, it doesn't download dependencies nor other packages. It doesn't require root permissions if you can write your current directory, this wouldn't store the files in the package cache directory.