This is a relatively inexpensive tool that will make your model making easier and help you avoid the need to be using door frames to draw lines. If you're new to model rockets, or returning after 20 or 30 years as the grown-up, here's the easiest way I can describe why you need this: you're going to be measuring and drawing lines on tubes. They need to be accurate, from end to end. There are plenty of other purposes for this item, but that will probably be the first difficult step you'll face. It's definitely worth having.