Page "Sequential logic" Paragraph 9

* The clock signal must be distributed to every flip-flop in the circuit.

As the clock is usually a high-frequency signal, this distribution consumes a relatively large amount of power and dissipates much heat.

Even the flip-flops that are doing nothing consume a small amount of power, thereby generating waste heat in the chip.

In portable devices which have limited battery power, the clock signal goes on even when the device is not being used, consuming power.