The red mark indicates how many perimeter-lengths the roller has traveled. The hook is for attaching ropes to pivot around poles.
A crop circle pattern 100 feet in diameter would have a surface area of 7800 square feet. Assuming that 2 sq ft are stamped per second, it would take little over an hour to make such a crop circle using this device.
Of course, complicated patterns would requiring calculations and surveying (not shown). Also, a board might be nice for trimming edges.