Why does React need a key on list items?

key helps React recognise which list item is the same item after reordering, adding or removing. That way a field's state and other related information stay with the right record.

If I show users, a good key is usually the user's stable ID. The position index can cause problems when items move within the list: state typed for one person can end up assigned to another. A key doesn't have to be globally unique across the app, only among siblings in that list. Generating a random key on every render makes it hard to keep an item's identity.