The world, and most movies, turn on big events. Events that are important or painful or epic (whatever that means) or profound in some way. But the big events are not the heart of most stories. It is the small events taking place around the big events that connect the reader to what is happening. If you see/read about an entire city being utterly destroyed, it has an impact on you. But how much greater is that impact if you see/read about people fleeing for their lives, trying to get out before it happens? How much greater is your grief when you see parents saving their children while sacrificing their own lives? How much more touching is it to see people rise to the best in themselves and risk their lives to help total strangers, when it would be easier to flee? Those tiny things make that big event so much more meaningful.
Now, that is an extreme example. Not all stories have such a huge, devastating event in them. Even so, the small tidbits surrounding your main storyline can support it. Seeing your big, battle-hardened warrior interacting with a child strengthens your story later when you have him fall in love and decide maybe he does want kids of his own after all. Seeing evidence of your villain doing cruel or nasty things to others in small things helps build the image of him/her as a mean person (think of our introduction to Gru in Despicable Me), so when they do something really heinous, the reader saw it coming. And if they manage to break out of that cycle and turn down a better path, we cheer all the more loudly that they overcame habits and tendencies they’ve had for most of their lives.
I’ve mentioned before that I write fan fiction. While I do write longer stories, I have a ‘series’ going that involves a lot of one-shots. These are more ‘episodes in the life of’ than anything else. But they build on the series, and add some dimension to a character or characters in the series. In one story, it focuses on a child cutting her own hair and involves the reaction by each parent to the event, and the perspective of the child as to why she did it. It was a small thing, but even so young, and even such an insignificant event in her life bolstered who her character was for the entire series. Would the series have been fine without that story? Yes, certainly. But such a small thing allowed an exploration into the thoughts and sensibilities of three main characters, and in ways that couldn’t necessarily be done in many other ways.
Was it critical to the Harry Potter series that Dobby developed such an obsessive liking for clothes? Not at all, but it endeared the character to us even more so that we cared more deeply what happened to him in the course of the remaining books. If we had met him in Book 2 and then not seen him again until Book 7, with very little mention of him, we would not care nearly so much about the outcome.
Details, even seemingly small and insignificant ones, can make a difference.