There is very little technical difference between RAID and AHCI mode, other than in RAID mode, the chips use different PCI IDs.
The reason is that Microsoft Windows has generic drivers which would take ownership of the AHCI chip, preventing Intel drivers being used. Switching to RAID mode changes the PCI IDs so that the Microsoft drivers no longer recognize the chips as AHCI chips and the Intel drivers (which have more features, such as software RAID) can bind to them.
Other operating systems typically do not recognize RAID mode at all (e.g. GNU/Linux) and require AHCI mode to be used, as RAID mode does not offer any advantages - the chips are the same with the same features.
Therefore RAID mode is merely a hack for Intel's windows drivers to make the AHCI chips artificially incompatible to AHCI.
This is why even laptops that can only use a single disk have a functional RAID/AHCI switch in their firmware - it's an unfortunate result of marketing, where RAID sounds better than AHCI, therefore even your laptop with only one disk has to have a RAID mode.
Another way to view this is as a king of copy protection - if the Intel AHCI chips used the "correct" PCI IDs then the Intel drivers would work with non-Intel AHCI chips, and people would be able to use them to have RAID features for these chips as well, as they are emulated within the driver - the RAID mode acts as a kind of dongle.