The purpose of weather vanes has long been solely to determine the direction the wind is blowing. The importance for knowing wind direction has its roots in early times before technological assistance on the prediction of weather has given us reasonable followings of weather forecast, by helping people guess at their weather and helping farmers to determine crop layouts to maximize their yields. Although there are many substitute designs for weather vanes from strings to flags, the most common association has always been to those metal or wooden designs that sit atop the highest point of a structure and rotate freely in the wind. In knowing what it does, how does a weather vane work?
Although weather vanes have their various designs and layouts that make them look as though they are favoring weight in a certain direction, weather vanes are primarily about balance and unrestricted rotation. All weather vanes are designed so that the entire mechanism pivots on a central axis. The importance of proper weight distribution is to enable the vane to freely rotate without having one side outweighing the other. With more elaborate designs for weather vanes, like those of the common rooster variations, having the detailed portion lighter than the base on the opposite side. Although they look unbalanced, they never are.
When the wind blows, the side with the larger surface area will be affected by the wind and position it in the direction away from the direction the wind is coming from. For this reason, many weather vanes are designed with an arrow on the side with the least surface area, and it points in the direction the wind is coming from. Thus, if the wind is blowing from the north, the arrow will point towards the northern direction. If it came from the west, then the arrow would point west. It's very straight forward, but only works properly if it is situated at the highest point of a structure, and that no other structure or obstacle, like trees or natural barriers, affect wind flow.
With a proper knowledge of the wind's direction, people throughout the centuries were able to predict the weather based on experience and knowledge of surrounding climates. For example, if they were located in a place where the north was cold, the south was warm, the west was wet, and the east dry, then they'd know to expect the related weather conditions when the wind came from those areas. In the case of farmers, they knew how to orient their fields and crops so the wind was more helpful at pollinating than it was destructive by blowing against rows perpendicularly. If winds were too strong from one way, then they knew to plant trees as wind blocks.
So while the workings of a weather vane helps provide nothing more than guess work on the weather and the directions the wind blows, at least it serves as a way of knowing.