When you’re planning your novel you can let the planning run away with you. You can have your main plot, but then you build. With sub-plots and character profiles, scene descriptions, timelines – it can be a complex business and the planning can go on and on, running further and further away from your main idea – your central theme. [And that’s before you even start writing.]
It may remain in your mind but with all the other things potentially floating around in there, it could run the risk of being lost in translation and your novel may be crammed full of several stories each with the potential of having their own novel each. And it could lead your novel to become one chaotic mess.
You can have sub-plots and several themes that you want to explore as the novel progresses, but to keep my mind focused upon what I actually want my novel to be about I write a few lines detailing the main plot – my main points. As sort of blurb if you will. This hopefully will keep my mind more focused on the story that I want to write; and can easily be referred back to if ever I feel my novel becoming a bit wayward.
So keep it simple. Write a few lines. Keep your mind – and therefore hopefully – your novel focused.