Jon Tidey wrote:no one mentioned grading.
Just looking for a straight answer. What do you need for a full screen view of the edit on a second screen?
Many, many editors use full-screen playback to show the current edit to directors, producers, and others so that their view isn't covered up by the editing interface. This literally goes back to Avid Media Composer in 1992. Hell, I literally had it with Final Cut Pro 3 in 2001. There's a different "feel" and impact when you see the show in context on a full-screen viewer, even on a 24" display or a 27" display. Even better if it's on a consumer-sized monitor.
Here's an example of an editing room in an actual post-production office:
The editor in front has two displays devoted to the editing software interface, a full-screen monitor with embedded timecode on the lower right, and then a big monitor above so that producers, the director, and others can have a clear unobstructed view of the show. The desk in back is probably for the assistant, backups, uploading, file copying, and transcoding while the work is being done. The editor has a laptop on the far left, probably for emails and personal stuff. This is how real editing is done, at least in NY, LA, London, and many other large cities for big shows, commercials, and films.