[imtringued]

Field effect transistors are basically a capacitor. They store energy.

If you switch a not gate's input from zero to one to zero and so on, the gate capacitance will have to charge and discharge. The entire idea behind CMOS is that if you have n and p channel transistors together, you can take advantage of the fact that electrons are more mobile than holes. Filling and draining electrons gives you a greater switching speed.

If the input stays the same, then the charge at the input inside the flip flop is the same as the charge inside the not gate. No charge differential means no electrons move, which means there is no ohmic resistance that causes the internal metal and polysilicon interconnect to heat up and less power gets lost and no switching obviously happens faster than some switching.

TL;DR If you randomize the data, you will constantly charge and discharge the capacitors.