A writer creates reader expectations by what events, characters, or aspects of the story he or she chooses to describe. When those promises are made, the writer is obligated to fulfill them. Not fulfilling promises will leave the reader dissatisfied, which is not the way to build a fan base.
The How to Train Your Dragon book series by Cressida Cowell begins with “There were dragons when I was a boy.” That is a promise. What could it mean and will I be grief-stricken when I learn the answer?
For twelve books, I followed the story and worried about how the series would end. When the end came, the promise was fulfilled. Without giving spoilers, let me say that I cried, but I was satisfied with the answer.