You could easily argue that the extra step isn't necessary for any of the input methods - once we have the image just insert it. However, I would argue that regardless of the person's preference, it's still an important step to confirm they provided the correct image.
At this step it hasn't yet been uploaded to Imgur's servers and they can potentially prevent leaking things they shouldn't before it instantly becomes more of a hassle to clean up. If we skip that step, it's already been uploaded and deleting the image is way more of a headache than saying "woops that's not what I wanted to paste."
Anyone can make that mistake, even people who hate the extra button click. There have been plenty of times that I've pasted something thinking I had something else in the clipboard.