Let's say I have an unused logic inverter gate, and somewhere close to it an output which is running at 10MHz.
If I tie the input of the unused gate to the output of the clock signal the earlier unused gate will now run at 10MHz, consuming a lot of power which is wasted. So it is better to tie the unused input to ground or Vcc, which will lock it at a steady state, which will waste a minimum of power.