Selecting text and middle clicking is so much faster than ctrl+c, ctrl+v. Having the two separate, allows you to essentially copy two separate things at once, and paste them separately, which is great for filling out forms.
Also, KDE provides an absolute gem of a clipboard manager, which stores all recent copies in to a list, from which entries can be browsed in order to recopy some text that you may have copied a while ago. IDK where you got the idea that Gnome apps have a different clipboard, as I have never experienced that. Maybe that is an old thing? Definitely not how it is now, thats for sure.
Copy/paste works on files / file-managers, images, text and HTML. This is not a Windows only experience. In fact, the clipboard manager even has little file icons, and image previews, for copied objects.