Useful if your gui breaks or if you uninstall all your terminal emulators

They are TTYs, they're like terminals your computer spawns at boot time that you can use. Their intended purpose is really whatever you need them for. I use them for if I somehow mess up my display configuration and I need to access a terminal, but I can't launch my DE/WM.

Each one is a virtual terminal and you can use them just like any other terminal. They exist because the easiest way to put some kind of a interactive display up is to just write text to a framebuffer and that’s exactly what your computer does when it boots and shows all that scrolling stuff. The different ones are just different framebuffers that the video card is asked to display when you push ctrl-alt-fnumber. You can add more or disable them altogether if you like.

Years ago my daily driver was a relatively tricked out compaq laptop and I used a combination of the highest mode set I could get, tmux and a bunch of curses based utilities to stay out of x for as much of the time as I could.

I mean, each vt had a slightly different colored background image, the text colors were configured, it was slick.

I used to treat them like multiple desktops.

With libcaca I was even able to watch movies on it without x.

I still use them when x breaks, which did happen last year to my surprise. If your adapter supports a vesa mode that’s appropriate to your monitor then you can use one with very fresh looking fonts and have everything look clean. Set you a background image and you’re off to the races with ncurses programs.