Linux

IT guy here. The CLI is not something I'd expect the average computer user to use at all. However, for power users and professionals it's a force multiplier at least, and a prerequisite often.

There are several reasons for this. Firstly, IT system and server administration, in the cloud or your own hardware, is often done via the CLI. This is because it's not that common or convenient to hook up every server in a rack to a monitor to click on stuff. But dialling into it remotely via SSH or even a serial port to perform bootstrapping procedures, troubleshooting and even routine management tasks sometimes, is very quick , easy and reliable.

The other main reason is automation. If I buy 10 servers to power my website, they all need installing and configuring a whole bunch of software, e.g. an Apache web server, DNS, SQL, Active Directory, AV, firewall, networking, and a host of other services. Now imagine doing all of that by hand. You don't even need to be a professional sysadmin installing server racks for a living for this to be important. Even if you run a couple desktop/servers/Raspberry Pi/NAS at home, they'll need updating, upgrading or replacing every once in a while. Having to click your way through everything every time you need to (re)configure them gets old very quickly.

GUIs are extremely poor at providing a consistent, predictable, automatable way to do things. They force you to do mostly everything manually and be present to supervise the whole thing. With the CLI you can script out pretty much any task and let it run in the background while you go do other things. I really don't see CLIs going anywhere anytime soon. I'd say it's actually the opposite. PowerShell was Microsoft's way of acknowledging this very fact years ago. The primitive Windows Batch scripting language wasn't cutting it for anyone, especially Windows Server users who had to painstakingly configure every Win Server install they did manually through a GUI wizard.