Yep, it's just a stick of wood. But, it is EXACTLY 10 inches long. And, while blunt straight cut on one end, the other is cut at an angle to leave a blade-like edge. Don't cut it too sharp or you will wear away its accuracy.
Purpose? Do you ever have to measure something that is between two 90 degree surfaces (like between two walls)? Just run your tape measure and eyeball the end measurement? NO, you will be disappointed with accuracy.
Instead, lay or clamp the stick on one end and measure to the other end. Just be sure to add 10 inches. It's easy. 27-7/16 is really 37-7/16. Dead on.
Have I messed up with this? Yes. Once I measured for a set of shelf boards to be cut to fit a rectangular cavity. Came back (30 miles from home) with some beautiful walnut boards, all of which were EXACTLY 10 inches too short.