I understand that Chinese directional complements are used to indicate direction of an action. I understand that when 来 is used after the verb there is some movement towards the speaker and when 去 is used there is some movement away from the speaker.
However, my question is when is the directional complement used because I see sentences in which it could be said that there is movement to/away from the speaker and yet there is no directional complement.
他们要到上海了。(Tomorrow they will arrive at Shanghai.)
When are directional complements used? Please answer generally rather than just answering in the case of the above example.