One of the common things we as Rails developers have to implement in commercial web apps is an area for receiving payments. PayPal has become almost a standard acceptable form of payment in the realm of e-commerce. So it would make sense to add this type of functionality to your web application.
For anyone who has ever tried and failed or those looking for an easier way than you may have implemented already check out this tutorial over at Railscasts. It is thorough and gives you clear instructions on how to get the checkout section of your site up and running! Any other ideas on how to add PayPal express to you rails app or any other form of e-commerce feel free to let me know in the comments section.