The Spread Spectrum setting is used to reduce the measurable amount of EMI a device may cause. In a computer, all devices (CPU, RAM, PCI bus, SATA, IDE, etc) operate at a multiple/fraction of the system clock. With all activities happening at exactly the same time, electo-magnetic radiation is emitted at a very narrow frequency. Since the energy is concentrated in such a narrow frequency range, it is measured as being more powerful than if it were spread over a wider frequency range.
Spreading EMI over a wide frequency range allows device manufacturers to certify their devices at lower EMI levels. This is because the FCC (USA), JEITA (Japan), and IEC (Europe) measure EMI within narrow frequency bands. The wide frequency at which a device causes EMI, the lower the levels of measured EMI.
I'm surprised it's disabled by default from a regulatory perspective, according to my research it would be enabled by default. It probably depends on the manufacturer and the country of origin. A technical reason for disabling the feature is below (number 3).
The benefit is that it allows a manufacturer to certify a device as causing lower levels of EMI.
The feature has the drawback of reducing the maximum clock speed of a device, causing compatibility issues with some RAID controllers, and reducing the bandwidth of SATA drives from 3Gps to 1.5Bps
I'd stay away from Spread Spectrum Clocking.
Disclaimer: I'm not a physicist, I just read a few Wiki pages.