It's a good practice to fall into and you can even do it on your own, though I like the idea of a group participating. What purpose does one week of steady writing serve, you might ask? There are five good reasons I always participate when I can:
1. It helps me to be accountable. We set a goal at the beginning of the week and then try to meet it. For some it might be as lofty as writing 10,000 words. Or it can be as small as 350 words a day.
Either way, there is a goal there, and if you work to achieve it, it furthers your writing. Even if you don't you will probably end up with more words written than you might otherwise have completed.
2. It makes me schedule a time to write. Sometimes days can be so hectic, but if there is a goal set and we want to reach our goals, we need to make the time to get the work done. And setting that goal makes me want to achieve it. And you only achieve it by sitting down and writing.
3. It makes me turn off my inner editor. Sometimes my writing slows down because I get caught up in a few paragraphs, trying to make them perfect before moving on. When you are participating in something like Book in a week (or in NaNoWriMo) there is no time for editing. You simply have to sit down and put the words on paper. Maybe they aren't as graceful as you wanted, but you're moving your story forward and getting something accomplished. The editing can come later.
4. It does good to see how the others are doing. I'll admit I can be very competitive and when I feel good about my 785 words written and I see someone else or a couple of other people double that, well, then the next day I am going to try as hard as possible to match them and their output. I want to write more!
5. There is a feeling of accomplishment at the end of the week. It feels good to look back and see how much got done, and often I have propelled myself past a knotty part of the story that was perhaps slowing me down. Writing fast can push me through that and suddenly I am on to the next part of the story and moving forward again.
So, even if you don't have a group to work with, try the book in a week challenge. Set a total word goal for yourself for the next week and then see how much you can get done. And then drink that glass of champagne and celebrate.