As a DM, one of my self-appointed tasks during the initial reading of an adventure is to identify the major plot points. These are vital because they're the "map" for what I can emphasize to make the adventure feel coherent. It's also the map for how the characters make progress.
If I forget to mention an important clue, then the game could grind to a halt. If I mention a clue, but fail to adequately connect it back to an earlier story thread that was hinting at it all along, then the story feels less like a story and more like disjointed vignettes. I feel like most RPG sessions that adults have tend to be pretty disjointed as it is. You don't have time to ponder and obsess over the minutia of the 2-hour game you played on Friday, because there's chores to do on Saturday and a lawn to mow and bills to pay and then a blur of work days, and on and on.
I want to do everything I can do to help my players recall why their characters are so determened to accomplish their current goal, and how they got to this point in the first place.
In my experience, it's rare for a large adventure to pull back and give you the big-picture overview.
What I really want from an adventure is a flowchart. I want a literal map of the major plot points of the main quest. I understand that every gaming group is inevitably going to tackle the challenge differently, but I find it less easy to adapt to that when I don't have a map of how the story progresses.
The main quest of any adventure generally happens no matter what the PCs do with their game evenings. If the DM tells the players that the castle's about to be attacked by giants, and the PCs decide to go for a beach holiday anyway, it's important for me as the DM to know what happens "off screen". And when the PCs return to the castle to find that it got plane-shifted away to safety, I need to know what clues to give them so they can find their way to the plane they're supposed to have been whisked away to as they heroically defended the castle. In other words, I like a map so I can reunite lost players with the storyline.
To the credit of most adventure's I've read or run, there's usually a plot summary at the start of the book. I have a few issues with this, though.
It's hard to remember the highlights once I've started planning out the details of just the next game session.
It's hard to see connections before I've digested the whole story.
It's difficult to reference dense prose quickly.
My own solution to this is to make a literal flowchart of the major plot points. I use the open source application Dia or Draw.io for this, mapping out just the actual "hand-offs" from one plot trajectory to another. If killing the goblin bandit is how the players obtain the stone key that opens the forgotten Dwarven stronghold, then I have an entry for "kill goblin, find stone key", and it points to "find Dwarven stronghold". In the stronghold, there's a puzzle the players must solve using the bones of fallen Dwarven warriors so that the spirit of the Dwarven king can appear and send them on a quest to the realm of the undead Dragon. Those get entries in my flowchart.
Should I forget to give them the stone key after they kill the goblin, I can refer to the flowchart as I'm trying to figure out what's next in the adventure, and I can make sure they learn about the Dwarven stronghold from a wandering Dwarf NPC who inherited the stone key from her father and is just about to head to the stronghold when she's ambushed and killed before their very eyes.
Should my players miss that there's an arcane puzzle in the crypt, I can ensure that the spirit quest giver appears to the cleric in a dream during the long rest they're taking after a harrowing battle with animated dwarven skeletons.
This is standard stuff for a DM, and it can be done easily without a story map. What I find, though, is that the overview helps me understand why specific elements of the story are significant. For instance, if the story had carefully laid out hints in chapter 1 that eventually the PCs would find their greatest victory by way of a goblin, it sure would be a pity that I swapped out a wandering dwarven adventurer for the goblin bandit they were supposed to have found the key on. Sure, the story progressed even with my change, but it's a missed opportunity to fulfill a story seed that had been carefully planted by the author. And maybe the PCs forgot about those clues, but would suddenly remember, and even see the connections themselves in a twisting and turning storyline. Those are the greatest moments in the storytelling side of an RPG, and I don't want to rob my group of them.
Photos by connor bowe and Averie Woodard under the Unsplash License, modified in Inkscape by David