The RAID On setting, in addition to enabling the controller to handle a RAID, uses the AHCI - Advanced Host Controller Interface. As I believe was mentioned earlier in the thread, the best known enhancement attributed to the AHCI is command queuing, a technique designed to execute instructions in the most efficient order. The AHCI also enables hot plugging on those computers having a BIOS designed to support it. Using the AHCI speeds up the data transfer to and from the hard drive by a few percent, but to be honest, the increased speed is small enough that most people would not notice. I've spoken to gamers who swear that the speed increase is enough to have a significant impact on their game play, but for someone like me who uses the computer more for data retrieval and word processing...well, I think you get the point.
In any case, as I stated earlier, the enabling of the AHCI is the only thing that is important to the RAID On setting in a computer that has no RAID. If your computer has only a single hard drive, or multiple drives but no RAID, you can look at RAID On the same as you would look at a setting that says AHCI, or in some machines, RAID Autodetect/AHCI.