What can a trilogy achieve that a single play/story cannot?

I'm writing an essay on the Oresteia, and the thesis I've decided to use is explaining what a trilogy can accomplish that a single story cannot.'


I need three points, any help would be greatly appreciated!What can a trilogy achieve that a single play/story cannot?
would the lord of rings ever have been so magnificent if it were just one single book. That trilogy is a work of supreme literature the books were magnificently written, and now the English speaking world is divided into two halves those that have read TLOTR and those who have not !What can a trilogy achieve that a single play/story cannot?
I guess a trilogy can portray the main parts of the story in a bigger depth?