Without nodes we might as well be reading a story or watching a movie. It's a valid pastime, and many people engage in it. But then again a lot of people complain about the choices made by novelists and script writers.
Roleplayers engage in a different pastime (they may read books and watch movies as well, but this blog isn't about those hobbies). Roleplayers engage in their hobby to make choices of their own and explore the consequences of these actions.
Some choose to explore certain settings through the personae they create. Possibly settings from movies or books, maybe even using the characters portrayed in those books (or thinly veiled facsimiles of those characters...but that's an entirely different blog entry).
Some choose to create their own settings and their own characters, using them to explore issues of emotional or ideological conflict.
But it's the choices that set the hobby apart from virtually every other pastime. Even computer roleplaying provides these choices, and this is one of the aspects that separates this genre from other computer games.
Which brings us back to the question of where nodes come from.
I've identified three sources for nodes. Different games use these sources in different ways, and different groups choose to emphasize different method of node development.
Nodes can arise through the scenario devised by the GM. Such a node could be pre-determined, allowing players to determine the outcome from a variety of possibilities defined by the GM, or a node could arise during the course of play if the GM believes that the story could benefit from a moment of tension or if they'd like to explore something specific about the characters.
Nodes can arise through the choices of the players. These nodes are less able to be predicted, and as a result many game systems place a lesser significance on them. They can be used to speed up the flow of a story (such as successful investigation attempts) or slow things down. But many game systems (and many GMs) prevent actions such as these from causing deviations to the actual story under way.
Finally, nodes can arise through the mechanisms. A game might kick into overdrive if the story tension gets too high, it might call for a specific type of test if a GM or player initiated node ends a certain way. It might shift gear when combat arises. An example of this might be Vampire's "Humanity" mechanism, a roll must be made to see if humanity is lost if the player makes a certain moral choice.
Every time a node arises, a choice is made. The story could turn to the left or the right, the situation for the characters could become better or worse, the story could continue, or it could come to it's conclusion.
"Traditional" games are typically defined by the idea that nodes may only be introduced by the mechanisms and rules of the book, or by the GM. Some would even push this further by saying that "Old School" games introduce nodes through the rules in the book or the predetermined scenario devised by the GM. In the old days it might have even been considered "cheating" if the GM introduced a choice to the players that they hadn't previously written (eg. adding a new trap to a dungeon if the players are going through it too quickly).
"Story Games" might be better defined by the idea that the players have free reign to add nodes at their whim. The destiny of the story is in the collective hands of the group rather than the single hands of the GM.
I'm still thinking through this idea of how nodes arise, and how the play experience changes according to the methods of node develoment, but this is a start.