Expect is a tool for automating interactive applications such as telnet, ftp, passwd, fsck, rlogin, tip, ["qemu"] etc. Expect really makes this stuff trivial. Expect is also useful for testing these same applications. And by adding ["Tk"], you can also wrap interactive applications in ["XWindow"] interface.
Expect can make easy all sorts of tasks that are prohibitively difficult with anything else. You will find that Expect is an absolutely invaluable tool - using it, you will be able to automate tasks that you've never even thought of before - and you'll be able to do this automation quickly and easily.