The benefits of leaving things undefined even in a traditional GM driven game are:
- Flexibility of plot from a GM point of view
- Ability to tailor the game to your players/characters
Especially in large campaigns, too much pre-planning can paint you into a corner. If you have a lot going on, as a GM you can easily get into story deadlocks and other problems if you have a highly rigid and predefined sense of the plot.
An important point here is that there's effectively no difference to your players between something you haven't figured out yet and something the players haven't come into contact with yet. There are three levels of definition in a game:
- Things the PCs have encountered (and think they understand, though
they may be wrong)
- Things they haven't encountered but you already
have a clear plan for
- Things they haven't encountered but you are
The difference between the latter two is only a matter of how inflexible a thinker you are. The first one is the only one you should be concerned about preserving. GM driven doesn't mean the GM can't change their mind behind the walls of the scenery on stage.
Did they kill your "big bad" ten levels too early? Well, there's nothing except some piece of paper that either a) you scribbled down while sitting in your underwear or b) some other guy scribbled down sitting in their underwear and you paid $20 for it that says the game is done now. Being open ended leads to more possibilities and more options for fun gaming.
You don't have to go in for newfangled indie hippie-dippie player input to tailor a game to your PCs. I actually find in-play player agency in the world/plot to be disruptive to immersion and prefer not to have it (as a player, mostly, it ironically bothers me less as a GM). But maintaining an interesting plot has a lot to do with subtly tailoring those events to your players - to make them more invested, to avoid deadlocks.
One of the PCs invested in heavy mind reading/control cheese? Well, let's make the BBEG a robot so that they won't just roll over him. What's causing the world storm (tm)? Well, the one scientific PC took a lot of points in botany and none in space science, so let's have the cause be something plant related instead of solar storm related so that it's more relevant to them.
In my games, NPCs frequently become more or less important to the plot based on whether the PCs engage with them or not. Which shopkeeper in that town is the cultist? You could just say 'the butcher! Done." Or you could make it whichever one the PCs interact with and like! Simulation and immersion are maintained for them, but narrative goals are furthered in the bargain. GM does stand for God in the Machine after all.
In one long AD&D 2e game I ran, the players were investigating a swamp and came across an abandoned plantation house I put in just for color. They got so interested in it for no reason I could fathom that I made it haunted, and the whole day's session turned into them crawling all over this house (and eventually flinging themselves out of the doors and windows shrieking, and the house disappearing). That's an example of changing what I had in mind - initially in my notes it was "abandoned house" and then the rest of the adventure was finding the goblins. But love of the game >> love of your GM notes.
But it didn't end there - I deliberately had left open and undefined what exactly the point of the Big Evil Underdark Aboleth Cthulhuey Plot that was going on was. Who was the prime mover and what were they trying to do exactly? Eh, too much work and then the risk of having the players get off track. I'd figure that out about 10 levels down the road. Well, when I brought that house from above back the players were more into that than any of the other leads I was spinning out. After a while, as I made real-time plot decisions with passive input from the players, it turned out that the house was that of a Cthulhu type sorcerer and naturally that was why it was haunted. And he was part of an adventuring party back in the day but something went wrong between them. And the PCs find out that they're each related to a member of the original adventuring party! And the party wizard is getting really interested in the Cthulhu magic in the house. An epic campaign, including very personal bits, was effectively created on the fly as the campaign went on.
And that's attributed to being able to take "abandoned house" and "there's some big aboleth cthulhu underdark plot," leave them open, and spin out the details in play.