The advantage for an author, who needs to make a living, is that a tried and true series will have readers pre-order amd order the new installment. From my point of view as a reader, I know I will meet "old friends" again, and see how their lives and relationships progress. I know that last sentence I read in the latest installment is not the last one forever, and that gives me a warm fuzzy feeling
That said, I find it irritating that trilogies are the preferred form of publishing in the dystopian genre. Is it because
The Hundergames trilogy was so successful? What's so magical about the number 3?! Every time I pick up a new dystopian that appears to be a stand-alone novel (thanks to Amazon's way of NOT showing you series names and installment numbers), it turns out to be the first in a trilogy. Ugh.