Linux

To add to what @bloodfart wrote, the history of TTYs (or virtual consoles) goes all the way back to the early days of computing and teletypewriter machines.

In the old days, computers were gigantic, super expensive, and operated in batch mode. Input was often provided through punched cards or magnetic tape, and output was printed on paper. As interactive computing developed, the old teletypewriters (aka TTYs) were repurposed from telecommunication, to serve as interactive terminals for computers. These devices allowed operators to type commands and receive immediate feedback from the computer.

With advancements in technology, physical teletypewriters were eventually replaced by electronic terminals - essentially keyboards and monitors connected to the mainframe. The term "TTY" persisted, however, now referring to these electronic terminals.

When Unix came out in the 70s, it adopted the TTY concept to manage multiple interactive user sessions simultaneously. As personal computing evolved, particularly with the introduction of Linux, the concept of virtual consoles (VCs) was introduced. These were software implementations that mimicked the behavior of physical terminals, allowing multiple user sessions to be managed via a single physical console. This was particularly useful in multi-user and server environments.

This is also where the term "terminal" or "console" originates from btw, because back in the day these were physical terminals/consoles, later they referred to the virtual consoles, and now they refer to a terminal app (technically called a "terminal emulator" - and now you know why they're called an "emulator").

With the advent of graphical interfaces, there was no longer a need for a TTY to switch user sessions, since you could do that via the display manager (logon screen). However, TTYs are still useful for offering a reliable fallback when the graphical environment fails, and also as a means to quickly switch between multiple user sessions, or for general troubleshooting. So if your system hangs or crashes for whatever reason - don't force a reset, instead try jumping into a different TTY. And if that fails, there's REISUB.

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

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.

If your system is borked sometimes you can boot into those and fix it. I'm not yet good enough to utilize that myself though, I'm still fairly new to linux too.

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.

Mostly for headless systems, servers and such. That and debugging, if your desktop breaks/quits working for some reason, you need some way to run multiple things at once...