Snowballs were used to defeat Snowverload during the 2014 Christmas event. They were gathered in stacks of 100 by right clicking him, and there was no limit to how much a player may have at once.
They were ranged thrown weapons, and were the only weapons that could harm Snowverload. However, players could not deal the finishing blow to him, which was only done if brought to Santa Claus at the Lumbridge crater.
Snowballs returned during the 2015 Christmas event. They can be used to pelt the snowmen the player built and also to pelt Snowverload and a few penguins, including General Pinchkin.