Mostly to make the chip seem faster then it is.

"This model is sometimes designated as having a 66 MHz processor,

since the clock input runs at 66 MHz. However, the processor itself

is not "clock doubled", so technically it is only a 33 MHz processor."

The above refers to the 33/66 designation of the Quadra 950

http://www.everymac.com/systems/apple/mac_quadra/stats/mac_quadra_950.html