It's a mechanical WIND VANE for self steering..
A Windex is an arrow vane to show wind direction and is totally static. Think of the old folks on the homestead with the barn. They look up and say - it's blowing from behind (aft) or over my shoulder (leeward or windward).
An anemometer is an electronic device that measures where the wind is coming from, it's speed, direction, and requires a display unit for you to understand what you are experiencing when the SH hits the boat.