Unix/Linux: What is the main advantage of creating links to a file instead of copies of the file?

The main advantage is not really that it saves disk space (though it does that too) but, rather, that a change of permissions on the file is applied to all the link access points. The link will show permissions of lrwxrwxrwx but that is for the link itself and not the access to the file to which the link points. Thus, if you want to change the permissions for a command, such as 'su', you only have to do it on the original. With copies you have to find all of the copies and change permission on each of the copies.