Some things have no command and no API. A preferences pane, a stubborn dialog, an app that only exists on the desktop. For those, open the Dock, pick the window you need, and it comes to the phone.
It is a live, interactive view rather than a picture. You get a pointer you can place precisely, the iOS keyboard forwards real keystrokes, and the clipboard travels between the two devices so you can copy on the Mac and paste on the phone.
Make the window fit the phone
A Mac window is the wrong shape for a phone, and panning around a letterboxed desktop is miserable. So you can reshape the window itself — Phone Portrait, Tall Column, Square, Laptop, Full Width, or a size you pick — and the app genuinely relays out at that shape rather than being cropped.
More than one person can watch
A mirrored window can be shared with several viewers at once. One person drives and the rest watch, so walking someone through something is a matter of pointing them at the window rather than starting a screen share.