One good use for /dev/tty is if you're trying to call an editor in a pipeline (e.g., with xargs). Since the standard input of xargs is some list of files rather than your terminal, just doing, e.g., | xargs emacs will screw up your terminal. Instead you can use | xargs sh -c 'emacs "$@" </dev/tty' emacs to connect the editor to your terminal even though the input of xargs is coming from elsewhere.