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.
For instance:
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.