That choice was largely about which drivers your OS included. The IDE compatibility mode meant you could use an older OS that didn't include an AHCI driver. The

That choice was largely about which drivers your OS included. The IDE compatibility mode meant you could use an older OS that didn't include an AHCI driver. The toggle didn't actually change anything about the communication between the chipset and the drive, but did change how the OS saw the storage controller built-in to the chipset on the motherboard. The newer host controller interface was required to make use of some newer features on drives, because IDE compatibility mode had no way to expose that functionality. (Later, a third option appeared for proprietary RAID modes that required vendor-specific drivers, and that eventually led to more insanity and user misunderstandings when NVMe came onto the scene.)