What is a CLI?
A CLI (command-line interface) is the plain way to drive a computer: you type a command as text, press enter, the program does the work and prints text back. There are no windows or buttons, just a prompt waiting for the next instruction. You meet it the moment you log in to a server, where that prompt, the shell, is the whole interface you get.
It can feel bare at first, because nothing is laid out for you to click. The trade is that everything is exact. A command means precisely what it says, and it does the same thing every time you run it.
Why run servers from the command line?
Two practical reasons. First, reach: a CLI works over a plain remote connection, so you can operate a machine in a data centre on the other side of the planet with nothing but a terminal. There is no graphical desktop to ship across the network, which is exactly what you want on a lean server.
Second, repeatability. Because a command is just text, you can save it, schedule it, paste it into a guide, or chain a hundred of them into a script that runs the same way every time. That is why setup notes on this blog are full of commands you can copy: a clicked button cannot be handed to someone else, but an exact command can.