Why? Because it means that the next time you sit down to write, you’ll know exactly how to start. Instead of sitting there wondering what might happen next, you’re ready to go. You build momentum immediately.
If you’re worried that you might forget, make a short note about it to remind you when you pick up the work again.
This one will take a bit of will power—part of you will want to finish that sentence. Fight it and notice how much speed and power this gives your writing sessions.
(For many more tips on creative time management, get my book, "Focus: use the power of targeted thinking to get more done." It's published by Pearson and available from Amazon and other book outlets.)