With a high concept at hand, you can get a general feeling for your story as a whole (thus, the sooner you get it the better). You can make the necessary adjustments before you are too far in.
Let’s say the first high concept you put together is this: Two friends find a relic, but an evil guy steals it and they set off to get it back before the world falls into ruin. A bit long, but it’ll do for our purposes. Regardless, we can get a better view of how the story will run.
Finally, I’ll get to the point of this post. In the high concept you can spot something (or the lack of it) that is pretty important to an interesting story. Interesting character conflict.
Currently, we have two friends (who will spend most of the book’s time together) and an evil guy whom no one knows. There will be conflict, yes. Interesting conflict even. Between the evil guy and the two friends. However, this is simply the times when they clash which will maybe make up a third of the story. Most of the time, it will be the two friends who try to get the relic back. They can bicker among themselves, yes, but it will not be as interesting as it can be. To use a better example, would Lord of the Rings have been as interesting if Gollum never joined the Frodo/Sam party? Without the constant conflict between Gollum and Sam and subsequently Frodo and Sam, the journey would have been a lot less interesting.
In our own example, we can do with a bit of interesting conflict. To do that, I’m going to shuffle things around a bit. Here’s a new high concept: Two friends find a relic, but when a guy from a rival village attacks them, one of them flees with it, intent on ruling the world, and the two enemies must work together to stop him. Also too long, we’ll leave it like that.
With the new high concept, we have more interesting conflict (the two enemies that must work together; the antagonist is the protagonist’s best friend) and secrets (the one friend hides his aspirations from the other; the enemies plot against each other while working together).
Some stories might lose too much to change it around like I did now, but an additional character or a change of relationship can make a great impact on the conflict which will make the story that much more interesting to read.
