Siri Shortcuts not only allow you to control your computer using your voice, but also enabled you to integrate your computer into your smart home environment, and control it using Shortcuts app, by creating elaborate workflows. You probably asking yourself why you need it... well it's quite simple, if you like automating things in your life, like for example you have HomeKit appliances in your Home, and you wish your to integrate your Mac into your home automation, Siri Shortcuts is a way to do it. John Voorhees spoke about this on AppStories.
You can add Siri Shortcuts for AirPlay (audio & display), App actions (Launch, Quit and etc), System actions (Sleep, Turn Display Off and Lock), and open websites. To add Siri Shortcut tap the arrow next to the action you want to use, to expose 'Add to Siri' button, define the phrase you want to use and that's it.
You can change the phrase or delete the shortcut from the app Settings ➤ Siri Shortcuts.