Before thinking about the chorus in Antigone specifically, it is important to realize that the Greek chorus was a common theatrical element that many playwrights of the time used, so the purpose of the chorus in Antigone will have its own specific purposes, but also serve the purposes of the theater in general.
In Antigone the chorus is filled with the people of Thebes—again, very common to the era. The chorus acts as a commentator on the play's action, and it sometimes offers advice to the leads, Antigone and Creon. For instance, the chorus provides some foreshadowing of the terrible consequences of Creon's actions, but Creon doesn't take this to heart until much too late in the play, and by then, Antigone and his son's fates are already sealed. The chorus in this play, as in others, sometimes provides hints at what is to come and suggestions on what can be done to change the course of events. In some ways, this is much like an audience screaming out at the stage, "No! Creon, stop!" And what does Creon do with this? He doesn't listen to the audience. Thus, he doesn't listen to the chorus either.
So, the comments are meant to guide the characters IN the play, but the chorus in any play is meant to provide a sort of emotional and/or response roadmap for the audience. If the chorus explains something to the audience, it might be because the crowd may need more context than the actions or dialogue of the characters provides. If the chorus is upset, it is meant to excite and rile up the audience. If the chorus is laughing, this could indicate the audience should be laughing, too, or that what the characters are doing is laughable (the chorus may even mock the characters, for instance).
As you can see, the chorus in Antigone, as with all Greek plays, serves several purposes; most importantly, they comment on the play as the play is happening, providing guidance to both the characters and the audience watching them.