Why do we have to use keys in React?

In React, keys are unique identifiers assigned to elements in a list. They help React's diffing algorithm efficiently determine which items have changed, been added, or removed. Without keys, React would have to re-render the entire list, leading to performance issues and potential bugs in component state. Keys also preserve component instances across re-renders, ensuring that stateful components maintain their state correctly. A common practice is to use a stable, unique value such as an ID from the data source. Avoid using array indices as keys when the list can reorder or items can be inserted/removed, because that can cause incorrect component updates.