I often find myself in the situation that I have an artwork and need to scale it (or resize the canvas) to specific dimensions which happen to be just one of the predefined ones, the ones available for selection when you create a new document.
Since I often don’t know the exact pixels by heart, I have to look them up. It would be so much easier when the drop down from the New Document dialog would also be available when scaling an image or changing the canvas size.
This way one wouldn’t have to open the New Document dialog just to check the dimensions again and then entering them in the scale/resize dialog manually.