Sometimes, weapons are made so accuracy is not a top priority. Roller fills that spot with simplicity. It will roll across the ground in whichever direction it was fired (or lands from) and will do a cool 20 damage.
What's better than a weapon you only have to aim moderately accurately with a likely chance of hitting? A weapon you only have to aim moderately accurately with a likely chance of hitting that does double the damage! Although it does double the damage, be careful because it doesn't travel as far on the ground.