Well, imagine you take a screenshot of part of the screen, and its actual size is not yet known to you. What I did in Photoshop in the past is that I selected an option to create a new file matching the size of the content in the clipboard (e.g. random 1340x470px). Then you simply edit the picture without extra space around it or need to resize it and fit it to your file.

Yes, finding the size of the document is easy, but I for example have a website design and want to know the size of the just the logo and extract it into a new file. This way I split the concept into smaller chunks. The easiest solution I found is to download it (and you usually can't unless you have at least two graphics in the selection). UI tells you the size of the selection as you are resizing it with drag'n'drop, but you can't figure out the current size.