It's useful for moving text around in a document, or even a long post. If you've typed in several paragraphs and decide that the second one really should have been the last, you can select it, cut it, and then paste it to the bottom of the post.
It saves the step of having to go back and delete the now duplicated text.
Copy & Paste is more common, and is mostly about getting text from one app to another, or from one text entry field to another.