The advantage of using PDT over IPN is that it enables you to track orders more efficiently. The best way to think about PDT is to consider where it fits in the three steps of a customer purchase:
Your customer selects what he wants to buy on your web site, and during the checkout process, all the order items are handed to PayPal.
PayPal processes the payment and confirms the sale to the user on the PayPal web site and via email.
Your site receives the order information via PDT, records the sale in your site's database (if you so choose), and shows the user a nice receipt, tailored to the order.
This is a much cleaner transaction experience for your customers than the process afforded by IPN, because they can see their order results immediately on your site. It also allows you to track only those orders that have been completed.
?Rob Conery and Dave Burchell