Imagine that files on your desktop didn’t have names. Instead, you’d refer to them by their order — the first file, the second file, and so on. You could get used to it, but once you delete a file, it would get confusing. The second file would become the first file, the third file would be the second file, and so on.
File names in a folder and JSX keys in an array serve a similar purpose. They let us uniquely identify an item between its siblings. A well-chosen key provides more information than the position within the array. Even if the position changes due to reordering, the key lets React identify the item throughout its lifetime.
list에 있는 어떤 값이 삭제될 때 순서가 바뀐다
→ 두 번째 파일이 첫 번째 파일로 변경될 수 있고 세 번째 파일이 두 번째 파일로 변경될 수 있다
→ item을 명확히 identify할 수 가 없다
→ key를 통해 명확히 identify할 수 있다