They transform the 5320 from a certificate-expired paperweight into a liberated, fast, and highly customizable machine. The extra 20MB of RAM alone changes the entire user experience, eliminating the spinning wait cursor that plagued stock Symbian.
Here is a comprehensive guide to understanding what RPKG and custom ROMs do, why custom variations are vastly superior to stock firmware, and how to find the optimal setup for your device. Understanding Symbian Firmware: The Role of ROM and RPKG nokia 5320 rom rpkg better