why key is used in v-for code example

Example 1: v-for

<ul id="example-2">
  <li v-for="(item, index) in items">
    {{ parentMessage }} - {{ index }} - {{ item.message }}
  </li>
</ul>

Example 2: vue js v-for array index

<ul>
  <li v-for="(val, index) in arr">
    <!-- Do something... -->
  </li>
</ul>