Designed as solid anchor points for placing compasses and dividers on the edges and corners of boards to mark curves, two of these Compass Guides from Walke Moore Tools allow you to make perfect circles or arcs anywhere on projects without fumbling to align a compass leg. (The round center guide is handy for marking curves on a face, without leaving a deep prick mark.)
The question is, do you need them? Truthfully, you can get by without them, but in some sense that’s like saying you can get by without a hammer if you have a heavy rock nearby. They’re the right tool for the job and they bring a functional precision to common layout tasks far surpassing the ad-hoc solutions most of us employ.