In the frequency domain a single frequency such as the external clocks are almost delta pulses with very high power.
By distributing the frequency over a 2%, 5%, 10% range from the average the maximum power contained at any on point in the frequency range is greatly reduced, whilst the average is kept the same.
As said before it's about the RF emmission. It's always said best to disable when overclocking but i don't know how much of an effect this actually has.