Diffing, and the actual reason keys exist

Comparing two arbitrary trees for the minimum set of edits is cubic, which is unusable. Every framework therefore makes two assumptions: two elements of different types produce entirely different trees, so on a type change it discards the subtree and rebuilds; and children are matched positionally unless you tell it otherwise. Those assumptions make the whole thing linear and are right almost all the time.

Keys are how you override the positional assumption. A key says "this is the same conceptual item as the one with this key last time", so a reorder becomes a move rather than a rewrite of every node in between. Without keys, inserting at the front of a ten-item list means the framework believes all ten items changed.

The correctness failure is the one that matters, and it is not about speed. Component state, uncontrolled input values, focus and scroll position all live attached to the DOM node or the component instance, and the key decides which item those belong to. Use the array index as a key, delete the first row, and the second row’s half-typed text is now sitting in the first row’s input. It looks like a data bug and it is an identity bug.