In some applications you require to change the clock rate of the MCU. consider that you implemented a LED pattern like in the video, and you require to change the speed of sequence, there are many ways to achieve this like using a switch to increase or decrease the speed, if your controller happened to have ADC then you can use that as well.
But if you have clock source in which you can control the clock frequency the programming part will be simpler. The following circuit I have used to produce required clock for 89S52.
The output of this circuit needs to be given to the 19th pin of 51 and 18th pin must be floating.