A situation is an atomic way of identifying an intended group activity. Usually, it's a place in the plot where a decision needs to be made or where there is an event with an unpredictable outcome.

It's easier to focus on the demands of each situation if they're considered one at a time. Planning for situations gives you the context for writing those pretty descriptions necessary for players to visualize the fantasy world. It fills the session with more content, and makes it seem more like you're telling a story.

If the game master thinks of the plot as merely a string of situations, the prepared material for a session becomes more managable. Since the division between them is so decisive, the session can be visualized as a list of situations (or an interconnected map if you're planning for divergences). The only alternative is to invent the plot in-session ("winging it"), which can never be as rich and beautiful.

1. It's not too long to be boring for those players who aren't enjoying the situation.
2. It's not too much material for the game master to create, especially considering that the direction taken by players may make any prepared material unusable.
3. It's short enough that the fun of the session won't be ruined if it doesn't go as planned.
4. It's long enough for the players to realize the nature of the situation, have some time to consider it, and for each to have their attempt and/or voice heard.