The middle of a novel is where the quicksand dwells!
Plotting a novel usually begins and ends rather well. It's that middle part that is challenging. Most of the time, you know how the book will begin and how it will end. You might also know or have a good idea of some key scenes. However, as you plot and you reach mid-point in your book, invariably your pace slows and you are faced with every writer's dilemma: Now what?!
When you don't do an outline or synopsis, this juncture can be daunting. You've been flying along and now you're stuck in quicksand. Have you read books where all of a sudden a secondary character shows up along with a subplot? This new development is introduced and dealt with in the space of, say, four or five chapters. How convenient! Well, that's one way to shore up a saggy middle, but it can also be disconcerting to the reader and play hell with your pacing.
However, the surprise subplot is better than chunks of introspection where the hero and/or heroine review what has happened in the book so far, reflect on it, share memories of their childhoods, and wonder what the future holds for them. This "soggy middle" rescue operation is always a poor choice. You will put your reader to sleep or the reader will skim over these chapters, and the charm of your book is lost. You will also probably receive less than enthusiastic reviews.
Outlining allows you to address saggy middles efficiently and effectively. Since you aren't in the throes of writing the book, you can stop and reflect on what scenes of interest you can add that will further the main plot, keep up the pace, and add to your characterization. The middle of the book can be -- and should be -- a build up of what's to come in the novel's final revelations, black moment when all seems lost, and wrapping up scenes. These important middle scenes should engage the readers, not put them to sleep. They should include surprises, insights, and carry your "red thread" throughout. Think of the Harry Potter novels. Did everything exciting grind to a halt in the middle part of those? No! Things heated up!
So, the next time you read a book where a Stop sign pops up in the middle and everything slows to a grinding halt, you can smile knowingly and think, Ah-ha! The writer didn't plot this one out well enough and found himself/herself in quicksand!
Writers can dig themselves out, but it does take planning and good plotting! And it is much, much easier to do at the outline stage than at the actual writing stage.
Good Luck! And keep a shovel handy at all times.