The clock has several uses:
It provides a 'tick' to synchronize the elements of the system.
It can be used to provide delays when needed
It ensures the system runs at the required speed
It can be used in charge pumps to provide voltages and power sources
There are other uses as well. Think of the clock as being like a conductor's baton to keep an orchestras musical instruments in time with each other. Without it, some of the instruments would play at their own speed and the result would be that they would play out of sequence.
Brian.