There are literally hundreds of payment gateways all over the world & it can becoming a big pain supporting them in your Joomla extensions. The Common Payments API is a standard way to integrate with supported Payment Gateways. The standard abstraction layer of the Common Payments API ensures that whichever gateway you want to integrate your extension with, you dont need to make any special changes in your extensions. Using this API in your extensions ensures that your extension will work with all supported payment gateways.
The Common Payments API was released as GPL open source code at the Joomla World conference in 2012 & has since been adopted by many extension developers. Future plans for the API include trying to get it included in the Joomla core as libraries in the platform/framework.