I don't really understand either! Looking at the tutorial I thought that it was to make trimming the block down easier,but then she said that her paper had shrunk...perhaps it's to make it easier to get all the strips lined up properly, as they are a bit wonky and I could see myself sewing them all together and then not having the right size at the end because I went too wonky.
Just my thoughts, hopefully someone who knows will be along soon!