Command line interpreters have the advantage that the user may issue a lot of commands in a very terse and efficient way. The downside is that one has to know the commands and their parameters. The graphical user interfaces were an answer to this problem. However for certain complex tasks GUIs are more difficult to use than a command line interface, because of the large number of menus and dialog boxes presented, and therefore a well designed command line interface is easier to use.
Examples of command line interpreters (not actually well designed, but grown historically and intensively used)