Candying ginger is one way to preserve what remains of the root after the required amount has been used for a recipe. If it just sits around, by the time it's needed again it will have shriveled and/or molded. On the other hand, if there is evidence of eyes that might sprout, it might be better to encourage them to do so and try to plant and grow some ginger. That's often successful and returns bountiful harvests.