Years ago my friends and I used to play a lot of Quake. Quake, and all the games using the iD’s Quake engine, allow you to pull down a console that let you have a quick command line to tinker with things.
Visor is essentially that same thing for OS X. It let’s you assign a key command to quickly pull down a Terminal session, and hit the same key to get it to hide.
And yes, you get to a new Terminal window pretty easily without visor, but having something that comes into view very quickly, and disappears just as quickly is great for doing things like tailing log files, or debugging an app in console mode.
And for some strange reason, you can run Quartz Composer files in the background, which is why you see floating rings in that screen capture.