It is common in actions when actors observe far-off happenings, such as armies massing in war or actual battles taking place, and in this way are witnessing the progress of the drama unfolding and in the dialogue are describing to others what they see.
Natural phenomena, too, such as the sun just rising above the horizon, or the description of stars across the firmament lend themselves to this treatment. However, the main object of teichoscopy is always that the process being observed is synchronous with the discussion taking place on stage, as opposed to the events being reported afterwards by messengers, etc.
Teichoscopy originated in the third book of Homer's Iliad when Helen describes to King Priam the Greek Heroes from the city walls of Troy. It is now a well-established technique in dramaturgy, representing, as it does the synchronous discussion of events, as opposed to action described by messengers after the event.