You can't always do that easily - you still need to mask areas. Plus this method gives total flexibilty in terms of coming back later to change it if you want. I know there are many ways to fix skies, especially since we can now adjust raw files in different ways and merge them together. But just pasting in a different sky doesn't do it unless you also fix the lighting in the rest of the scene. With this method, you are improving what you have so you don't have to worry about the lighting not matching the sky. For some reason my example photos are missing on the tutorial - I had a very complex sky scene with a bridge in the middle so it wasn't a simple cut and paste.