About mt_rand of an array

I have a code which selects a random url from a folder like this:

<?php
  $urls = glob("videos/*.php");
  $random = mt_rand(0, count($urls) - 1);
  header ("location: ".$urls[$random]);
  exit;
?>

I had help with the $random function, but did not receive an explanation for it.

The code works perfectly. But I'm wondering why it looks like this:

mt_rand(0, count($urls) - 1);

Instead of this:

mt_rand(count($urls));