I was wondering if there are any difference between
$autoload['helper'] = array('file');
The difference? If you have a small website, it's negligible. But if you want to squeeze your website to the last bit for performance, it's better to load those helpers only where they are needed. Loading them is still an I/O request, several function calls and includes - which is overhead, noticeable in large scale.
codeigniter is focused on speed, that's why most of the helpers you rarely need in a page are loaded optionally (not to mention an optional model).