what does this accomplish of Collection "map" method

[wdchui]

the office source code (https://github.com/laravel/framework/blob/8.x/src/Illuminate/Collections/Collection.php)
/**
* Run a map over each of the items.
*
* @param callable $callback
* @return static
*/
public function map(callable $callback)
{
$keys = array_keys($this->items);

    $items = array_map($callback, $this->items, $keys);

    return new static(array_combine($keys, $items));
}

i think the below code can achieve the same result and i have tried many times:

public function map(callable $callback)
{
    $items = array_map($callback, $this->items);

    return new static($items);
}

array_map does not change $this->items keys, so what is the purpose of this design?

[wdchui]

this is because some callback need to get $this->items keys, by default callback can only get the value