You use grab mainly to reposition items in the world without moving them to inventory.
when in the world using is for activating an items subfunction. for food its eating, for the drive box its taking the lid off.
when an item is *held* in your offhand, you use its subfunction by clicking. this way you can use food without worrying about dropping it, for example, or to equip a tool so you can use it's subfunction ON an item. since you cant use food any other way than eating, that's all it lets you do, does that explanation help clarify the control sceme at all? i would not enjoy this game much if its interaction mechanics were simplified, the depth of what you can do in it is what i play it for X3