What is the application of a bounding box of a text in PHP *imageftbbox*?

In the example of the [manual](http://www.php.net/manual/en/function.imageftbbox.php), it was used to determine the coordinates for the *imagefttext* function. Is it necessary? Maybe I am missing something here.

```
...

// First we create our bounding box
$bbox = imageftbbox(10, 0, $font, 'The PHP Documentation Group');

// This is our cordinates for X and Y
$x = $bbox[0] + (imagesx($im) / 2) - ($bbox[4] / 2) - 5;
$y = $bbox[1] + (imagesy($im) / 2) - ($bbox[5] / 2) - 5;

imagefttext($im, 10, 0, $x, $y, $black, $font, 'The PHP Documentation Group');

...

```