A Render Texture asset file is a special asset file that can store images sent from cameras. Cameras usually render (that is, send their images) to all or part of the screen. However, as we've seen in this recipe, interesting effects are possible when we send a camera output to a Render Texture and then have a 3D object in our scene that displays the current image in the Render Texture asset file.
In this recipe, we've simulated a kind of mirror effect. Another common use of cameras and Render Texture asset files are simulated closed-circuit TVs, where a Cube (or a more detailed model of a TV monitor) can display what is being viewed in some other part of the scene by a camera. So, you might have a game where a player is in a control room, changing between which cameras they are looking at, to find out the location of enemies or teammates.