The primary function of bum rolls was to shape the lower part of a woman's body to fit the fashionable silhouette of the time. They added volume to the hips, creating a more rounded and exaggerated shape. Bum rolls also helped to lift and support the back of the skirt, maintaining its desired fullness and shape. Additionally, bum rolls helped to alleviate the strain on the lower back and hips caused by the weight of the skirts.