Wait, NO. The GM drives the plot! So what it main about the main characters? Wait, NO. The GM drives the plot! So what it main about the main characters?
The DM sets the boundaries and focuses the improvisation.
A lot of people have a hard time improvising (role playing) without boundaries. It's much easier when focused on a clear problem. The DM provides this focus.
That's also why railroading is still a topic. Without any railroading, it's hard to get a game off the ground; and a bit of railroading can often help a game that's going slow. On the other hand, once the player's are engaged and their creativity is sparked, it's important to allow them off the rails.
That also goes for writing. Sometimes you are stuck and just can't find a good way to continue - maybe you think "oh, it should be like this, somehow, but how...?" and you see lots of options and they all suck. Add in a new constraint (best one that you, the author, don't really like), and often this inspires you again. Human problem solving in action.
So in essence, I think the GM drives the plot to help the players be creative, and focuses their creativity by providing constraints.
