In the beginning ... hmm... perhaps not that far back. I got started on my history kick after reading the appendixes to the Return the King. In addition I had a big three inch thick book called the Timeline of World History. Many of my friends were bored to tears by the format but I could see the ebb and flow as I read the various timelines and king's list. When I starting writing setting background I naturally made my own timelines and kings list.
And my friend were still bored to tears by it all although they like playing in my campaigns.
As time went by my GMing and writing improved and what I realized that it isn't about history but about the people who make up history and the more importantly the people in the "present" where the adventuring is at.
The choices people make now are made in the context that is created by the choices of people in the past. Because it is impossible to detail every person's past for a game I instead focus on the cultures. Then make each NPC a variation on that culture. So the histories I write today are the history of the cultures that make up the Majestic Wilderlands.
The reason for each aspect of a culture can be found in it's history. Much of this will be pure color but here and there are nuggets that can be exploited for adventures in the present. And it serves as a good guideline for creating NPCs of that culture.
The complexity comes from the fact in real life people are not of "one" cultures. Instead a multitude make up the context in which a person makes their choices. Social, Religious, Nationality, etc.
To me the best use of writing histories is to explain why the people of your setting are the way they are. The NPCs won't know everything you now. Some facts will be lost despite being part of the chain that leads to the present. Doing this will help make your setting come alive and that the people have their own lives and agendas that didn't come out of nowhere. By having a history the player can discover what underlies the people of the lands in which they travel and use that knowledge for weal or woe.
AD&D Session 2: White Plume Mountain (Part II)
52 minutes ago