The "blurb" is the back cover material for your book the selling points which will get people to buy the book. If you write the blurb before you write an outline, you're guaranteed not to wander off the track as you write your book.
Writing the blurb will keep you on track as you write your proposal and your book. I've included two sample blurbs from my published books to show you how to write a selling blurb.
'Writers starve in garrets.
Many professional writers make incomes that would make doctors and lawyers envious. Most make reasonable incomes. If you decide to make a career of writing nonfiction books, the major benefit is that if you choose your book's topic with care, your book can stay in print for many years. For each year that your book's in print, you get two royalty checks. Let's say that you write two books a year for five years. At the end of the five years, if your books all stay in print, you'll be getting ten royalty checks a year. These ongoing royalties are your nest-egg, profitable investments in your future.