With the help of cec-client (part of libcec), your Raspberry Pi can control a device that supports CEC, like a TV, connected via HDMI. You could power the TV on or off, switch the active source, and more.
This should work with any Raspberry Pi version or model, including the original Raspberry Pi, as well as the latest Raspberry Pi 4.
A possible use case for this would be to connect to a Raspberry Pi via SSH and send a command to power on or off a TV connected to it via HDMI-CEC. Or you can use the commands to power on TV and make the CEC adapter the active source in a script, so that when you open some application on your Raspberry Pi, the TV that's connected to it via HDMI-CEC powers on and switches to your Raspberry Pi HDMI source. I'm sure you can think of various other use-cases.
CEC, or Consumer Electronics Control, is a feature of HDMI which allows controlling devices connected through HDMI using a remote control. For example, CEC is used to get the play/pause buttons on a remote control to control playback on a device connected via HDMI. Or when you play a video on a Chromecast with the TV off, and the TV automatically powers on and switches to the Chromecast source.
Most modern TVs and AV receivers should support HDMI-CEC. However, it's worth noting that you may need to enable CEC in the TV settings on some models. CEC may have a different name, depending on the device brand. For example, it's called Anynet+ for Samsung TVs, EasyLink or Fun-Link for Philips, SimpLink for LG, and so on.