This wasn't about how the SATA controller talks to the drives, but about how the SATA controller is presented to the operating system. The IDE/legacy setting wa

This wasn't about how the SATA controller talks to the drives, but about how the SATA controller is presented to the operating system. The IDE/legacy setting was useful at the time because Window XP lacked AHCI drives on the install media. This compatibility setting simplified the installation of Windows XP on newer hardware which was very common in the DIY space at the time it was introduced. Afterward you could install AHCI drivers and switch to AHCI mode for improved performance (iirc AHCI was required for NCQ).