When you will equip some new gears you will have the option to use the old gears as material to give you some more boosts to your power. All that you have to do is head on to the Buddy tab which will open the interface of your small buddy. On the bottom side of the screen you will see the button which will let you Devour your old gear. Tap on that and you will be able to feed your old equip to your buddy. Do that and it will give you a small power boost!