What is the pen holder/attachment used for?

If you're just getting started with cutting in particular it is a valuable resource for learning without wasting expensive vinyl. You can use plain old paper and the Pen/penholder- and draw out the images that you are going to cut- before you cut them. It is also a great tool for troubleshooting. If your cutter starts giving you inconsistent cuts or cutting related issues, you can use the pen as a "control" device to determine if it's a problem with the cutter itself, or if it's just your blade/blade-holder that is failing. I could really go on, but the long and the short of it is this: It is a useful tool and it is highly recommended.