essentially think about it this way:
if you start doing something over here NOW, and then a bit later you start doing something else over There NOW, and then you want this third thing to happen right about....NOW....
well having a good word clock will make sure that all of these events happen exactly when you want them to. it's like a timer that keeps everybody updated on what's going on. without it, you'd be in a real mess trying to do synchronous recording.