There are some scenes in a movie, play, or story that the readers want to see and will feel disappointed if they don=t see them. Sometimes these scenes are not strictly necessary to that elusive beast, The Plot, but that doesn=t matter. Readers will feel cheated if they=re not there.
Consider the end of THE RETURN OF THE KING. It would have been possible for Tolkien to leave out the bit where the ring goes into Mt. Doom. He could have kept the point of view on the battlefield with the other main characters, waiting and hoping C until suddenly, off in the distance, the volcano blows. Someone could cry AFrodo=s done it, he=s destroyed the ring!@ I suspect a great many readers, myself included, would have muttered something most unflattering to the author at that point and perhaps even flung the book across the room.
Obligatory scenes can occur at other places in a book than the end, of course. Another example from a fantasy novel: two characters are riding toward an important destination. Alas, the only road runs through the mountains in a pass known to be infested by bandits. What=s more, the enemies of the two characters are probably waiting there to ambush them. They head into the pass. Chapter Break! They are riding out of the pass, quite beaten up, to be sure, and talking about what a stiff fight they had, there in the pass. Why the editor allowed this writer to get away with this lapse, I don=t know. I sure wasn=t impressed enough to read another book in that series.
Nor does the obligatory scene have to be a large or violent confrontation or action sequence. It can be a simple emotional moment or a conversation. For instance, in real life history, Mary Queen of Scots and Elizabeth I never met. On stage (Sardou, I think) and in many movies, they have met, because hell, they really should have, and the audience wants to see it.
Katharine Kerr spent her childhood in a Great Lakes industrial city and her adolescence in Southern California, whence she fled to the San Francisco Bay Area just in time to join a number of the Revolutions then in progress. After fleeing those in turn, she became a professional story-teller and an amateur skeptic, who regards all True Believers with a jaundiced eye, even those who true-believe in Science. An inveterate loafer, baseball addict, and rock and roll fan, she begrudgingly spares time to write novels, including the Deverry series of historical fantasies or fantastical histories, depending on your point of view. She lives near San Francisco with her husband of many years and some cats.