General Importance of Timing
Clocking (or timing) is essential because digital signals are evaluated or read by the voltage on the line.
Each piece of equipment needs to know exactly when to read the voltage. Figure 1 depicts an ideal digital signal. (The 0 reading equates to binary 0, while a V value equates to binary one.) If your clock told you to read the signal when the signal was at time t1, you would record that signal as a 0. While reading the signal at time t2 means you record the signal as a 1. If all devices are synchronized, they will transmit and receive the signal correctly. If two clocks are out of synchronization, you can end up with one person reading the signal at what it thinks is the correct time (t1), but is actually time t2, giving an incorrect value.
You would like every device in your network timed by the same clock source. Either timed directly or timing derived from the same device that provides clocking for the entire network. Since you derive timing from a "Master" clock, all the devices in the network will be synchronized.
Traditionally the wide area carrier provides a Stratum 1 clock source.