When you take a look at a published book, there are three components to what you're seeing.
1. The cover
2. The layout of the text
3. The content of the text
The Cover - this is the first thing you see when you pick up a book. Are the colors attractive? Is the picture well-executed and does it match the story? Is the title memorable? Is the balance of elements correct? They say you can't judge a book by its cover, but I have to say this. I've noticed a lot of crummy books with great covers, but I don't often see a crummy cover wrapped around a good book. Although I have seen exceptions, generally speaking, if the publisher hasn't taken time to create a good cover, they haven't taken time on much else, either. Covers really do make or break sales.
The Layout - This part of the production process is really noticeable when it's done wrong, and invisible when it's done right. Ideally, the typesetter will create a layout that is pleasing to the eye, that leaves enough of a margin without too much of a margin, and will start all the chapters on the right side of the page. The text will be tweaked to eliminate hyphens at the ends of lines as much as possible, and won't squish the letters too much or spread them out too much to create this outcome. If the typesetter has taken care with their task, the reader doesn't really notice anything and just enjoys the read. But if the words are squished or there are hyphens hanging off the end of each line, the reader's eye doesn't feel comfortable, and that doesn't present an easy read.
The Text - The third component is the text itself. This is the part that started out as your manuscript, that has been edited and reworked and honed and prepared for this day. You wouldn't want a lovely cover and awesome typesetting to end up with a bumpy story, so taking care of this part of the process is crucial. Does the writing flow? Have the repetitive words been taken out? Does each word really mean what you want it to mean? Do your comparisons really compare? These issues go deeper than just characterization or plot line - there are nitty-gritty, finite details that separate a good story from a great one. Are there six "s" words on line two? Did you mean to say "definite" or did you mean "define?"
Each of these elements are crucial to the overall quality of the book. You can't have one without the others and create a product that rises above the competition. The cover draws the reader in and sets the stage for the story. The quality of the text creates the production on the stage, and the layout acts as the behind-the-scenes crew, making sure that everything runs smoothly so the audience out front never knows just how it happened - they just know they were treated to an incredible performance. Each element is a team.