Mind you, writers need readers after publication too, but as a book is being written a good reader is crucial to the process. I can't imagine writing without feedback. It's how writing works: we write; they read. This vital connection is necessary throughout the writing of a book, from first draft to last.
I've mentioned that Annie is reading Xmas Carol (update: Annie's done and she really liked it!). As she read the book, our intrepid full-time commenter sent me messages about what didn't work for her (along with effusive messages about what did work). This is so helpful.
As a writer, you never know how readers are going to respond to your words. You have some notion about how they might react but until an actual reader gets his or her hands on your book, you're working in the dark. Don't get me wrong: it's fun in the dark! But sometimes we need to look out, and readers provide a window.
My sister Maria has been my main reader through the writing of three books. She notices inconsistencies and contradictions, which is very important. It's a gift to me when a reader notices something is wrong. Now I can fix it!
I completely altered the trajectory of my second sci-fi book (more about this in the coming months) to allay Maria's concerns. It's a different book now directly because of her input. With a well-aimed critique, I'm able to rework a story in a way that makes it better -- sometimes much better. Annie did this for me as well with Xmas Carol. Her comments were spot-on and I'm going to rework some things as a result.
The only way a writer can get reader's insights is by having readers. I'm lucky to have helpful souls like Maria, cousin Carmine, Annie and five or six other readers in my life. Constructive input makes a big difference to one's writing.
A PS to budding writers: Let someone in. I know you're afraid to show your work to others but give it a shot. And then put your ego aside and listen to what your reader tells you. Just listen. Hear every word your reader says and consider the possibility that the reader's perspective may be accurate. Doing this may shake you up -- but a good shaking-up can be helpful. Don't be afraid. Hand your work to someone you trust and then cross your fingers. Heck, maybe you'll get an excellent reaction. You never know.
Waiting for the reviews is part of being a writer. So get yourself some reviews. If you pay attention to what your readers tell you, I promise you'll become a better writer.