Vlock is a console screen locker.
You can use it inside an terminal emulator or TTY. Of course running it on a terminal emulator doesn't make much sense since people would still be able to access the rest of your computer if you wouldn't lock it. One reason could be that you log in as root or ssh into another computer and want to log that session.
A more common use is, in case you don't use a login manager like for example GDM/Slim and log into X from TTY you can use it to lock your screen after logging it. So another person can't just kill X and use your terminal while you are gone.
root # startx && vlock