If you read the tutorial then I’m sure you read this fragment:
That’s the answer to your question. And if I may add: that comment is made for a reason. You are free to try it with less than that amount, if you ran into issues then you will know why. I’m not sure why we are talking about this, since it was also stated before that your unit has 3GB of free space…
About why is that free space needed is for package processing, remember your upgrade USB device has compressed files only.