question: why the parse function returns also an id?

I'm looking at the code, in particular to the parse function.

```js
let id = 0;

function parse({ htmlRoot, js }) {
  let result = {
    // increment the id every time we call parse, so we can use
    // it for react keys, when iterating over targets
    id: ++id,
  };
```

Is the id important to the parse function? I looked at the code but I didn't found the place where it is used. 