React Documentation says:
"You need to give each array item a key — a string or a number that uniquely identifies it among other items in that array."
"Keys tell React which array item each component corresponds to, so that it can match them up later. This becomes important if your array items can move (e.g. due to sorting), get inserted, or get deleted. A well-chosen key helps React infer what exactly has happened, and make the correct updates to the DOM tree."
But have you wondered what really happens when using index or a unique ID as the key in a bit more visual detail? Check it out here.