When Ian's Credit Class work is integrated into the project, there would have been too much information shown to the customer on this one page.
Originally the textarea box was moved to the Checkout Confirmation page however due to the flexibility of the modules, depending on which module is installed and used the information could have been lost if the form on the confirmation page was sent to the payment processor server.
It is possible to have the textarea box on the Confirmation page however the modules requiring to make a request to another server via HTTPS POST would need to be done via Curl or using native SSL features in the upcoming PHP 4.3 release - something which would have broken compatibility between servers.
This is why it was moved to the Checkout Success page.
By using Curl, the form method on the confirmation page would be taken to the processing page (checkout_process.php) where the request to the payment processor would be done in the before_process() payment class method.