I recently read Kim Stanley Robinson’s Red Mars. (Spoilers ahead). I really liked the book, but I want to especially bring up a marvelous solution used by the author:

Around midway through the book, a miracle treatment that prolonged the life of treated individuals was introduced. The treat was introduced kinda like a side story - one chapter just jumped to the scientists making the discovery. There were no prior hints that such research was going on and no real indication that the setting (colonization of Mars) would somehow advance such research.

But brilliance of this invention was twofold: first, it allowed the author to keep the original cast (First 100 settlers) in the forefront of the action while still allowing the settlement process to take years and years (one could argue that the development was still too fast). At the same time it gave a plausible in universe explanation, why it made sense to keep sending more settlers to Mars. Too major story beats explained by rather simple addition.

Got any other examples of stories where relatively minor side stories / background details / world building choices help the story along massively?