Sometimes you want to compare parts of a text that is not an own file. KDiff3 also allows you to paste text from the clipboard into the diff input window that has the focus. The diff analysis happens immediately then. In the open dialog you need not specify files then, but just close it via button.
You can also use drag and drop: Drag a file from a file manager or selected text from an editor and drop it onto a diff input window.
What's the idea? Sometimes a file contains two similar functions, but checking how similar they really are is a big effort if you first must create two files and then load them. Now you can simply copy, paste and compare the relevant sections.
Currently you cannot drag anything from KDiff3. Only dropping in the diff input is supported.
Some editors still interpret the drag and drop into another program like cut (instead of copy) and paste. Your original data might be lost then.