In Antigone by Sophocles, as in most Greek dramas, certain types of event are narrated by messengers rather than portrayed directly.
First, violence and death normally take place offstage and are narrated rather than being shown directly. This is a convention consistent across all ancient tragedy, and may be for religious or ethical reasons.
Next, messengers are used to maintain unity of place. This is a practical matter of dramaturgy. It was only with the invention of the proscenium and the curtain that it became possible to change the dramatic location of a play out of the site of the audience by shifting around furniture and backdrops behind a closed curtain. In the open theatre of the Greeks, this was not possible, and thus events occuring in locations other than the main action of the play (e.g. the cave in Antigone) are reported.