Rengeki is used to chain to combo after a monster turns purple. By doing so you can gain extra expierience and items after the battle. It acts as a bonus. In later volumes Rengeki is altered to allow faster weapon changes. You can also use it to interupt monster attacks by speeding up to assist your party members. Try not spam it because though you can move faster your action gauge drops and you are unable to at as you like. The skill menue does have a purpose so don't forget about it.