"Why would anyone do this?"
. It allows these systems to recognize and use the full capacity of modern 3TB+ hard drives by adding support for the GUID Partition Table (GPT) Paragon Software Key Compatibility Requirements
GPT Loader modifies the storage stack at the kernel level. On XP, this means no Driver Signature Enforcement . Ensure you have a system restore point before installation.
: The current version cannot be used to host the Windows OS (boot drive) on a 2.2TB+ partition. Key Features GPT Driver