rmdir is usefull when you want to remove all empty subdirs, without touching files or directories with files. Just do 'rmdir *' and you're done.
I like using rmdir even for individual directories that I think are empty just so that I don't accidentally delete any files that I did not expect to be there.
I use it whenever I expect a directory to be empty and want an error if for whatever reason it isn't. It's a bit niche, but good to have in your toolbelt.