I've been following this practice for years, and it is great not only for keeping your story on track but in being able to see the "20,000 foot view" of your story and be able to pick out the premise and what is fresh and exciting about it for blurbs, ad, back cover copy, etc. When you are finished writing the book and know all of the minutia, it is much harder to pick out the most important points to cover in your blurb.

I've recommended it to many other authors over the years.

And it is possible even for a pantster who only has the vaguest idea of what they are going to write about. Know something about the character? The conflict, challenge, or journey they are going to face? That's all you need for the beginning of your blurb. "As [character] faces [challenge], he [thought, action, or emotion]. Things get worse when... Will [character] succeed in...?

You'll polish it up after you finish your first draft, so it's not written in stone. Say it in your dramatic announcer voice and get excited about it!