clock signals in computers and machines
"Time is what keeps things from happening all at once." - John Wheeler (1911-2008)


You don't want to run things unorganized. Everything in a microcontroller happens consecutively, in discrete steps. Getting a byte from RAM to a register: put address on address bus. Tick. Assert RD line. Tick. Latch data from databus into register. Tick. 

If you would do this all at the same time, you would latch the data before the RAM had time to put it on the bus. This kind of limitations occur all the time, and they are the limiting factor for a controller's performance.  

Note that some mechanical systems use a mechanism with a similar function:  

  

A watch's escapement also ensures that things happen timely, and even in an analog clock, in discrete steps. (Personally I count the escapement, along with the zipper and the wood screw, to some of the Truly Great Inventions.)