Modern CPUs run faster than the motherboards do. Thats where clock multipliers come in. If you have a 2GHz CPU and a 1 GHz bus, your clock multiplier is 2. If you have a 2Ghz CPU and a 666MHz bus your multiplier is 3.
CPUs process data faster than they can communicate. While waiting for the bus, they can and are doing work.
Higher frequency CPUs do make your computer faster, but as the CPU frequency goes up and the bus speed stays the same, the less efficient if becomes. This is because the CPU is waiting for the bus to give/take data.
So if the CPU is working while its waiting for the bus, you are using more power, but gaining little benefit.
Programs like Intel SpeedStep, lower the frequency of the CPU when not under heavy load, so it consumes less power. You might not notice a speed difference, because the lower CPU frequency might be more than capable of handling the current workload.