Good question!
From Wikipedia:
History
Bells of this type were developed centuries ago for fastening to harnesses used with horses or teams of horses.[1] Typically they were used for horse-drawn vehicles, such as carriages and sleighs. The bell was designed to make sound whenever the horse and thus the vehicle was in motion. The purpose was perhaps to herald the approach of someone important, or likely to warn pedestrians of the vehicle's approach so that they might step aside to avoid collisions and potential injuries. This was especially important for sleighs, which otherwise make almost no sound as they travel over packed snow, and are difficult to stop quickly.
