You can listen to your cast of characters at any stage in the writing process, but it might be especially useful near the end when most of the story is fleshed out. It can be a good check of your story logic and add a last ingenious layer of polish.
Everything in your book works to deliver your message — the main point of your life story that readers will remember long after they’ve finished your book. Just look at the titles of a bunch of memoirs to see how important this is.
Authors want to manipulate readers' thoughts — this is why we read, after all. We love getting swept away into new worlds, with new people and ideas. Getting a reader's thought map is a great way to hone this skill.