The reason I’m so obsessive about formatting when I give manuscript advice is that it’s perhaps the only part of the publishing process where the author has complete control. For example, let’s say you write a great Mystery-Thriller set in 1975 about a young girl who gets hit by a train while running from a strange man:
There are a lot of reasons why a publisher or agent (or any other reader) might not be interested in this book: maybe they don't like period novels; or maybe a young girl being chased by a menacing man is too scary for their taste; or maybe they’re okay with the girl being chased by the man, but they don’t like that the girl dies; or maybe they find your writing style too flowery, or too dry, or too graphic, or they don’t like that the narrator is first-person, etc.; or maybe your name is hard to pronounce and the reader has a weird problem about that; etc. etc. etc.
My point is that you, the author, have no control over whether or not one individual reader is going to like your work.
However, you DO have control over whether or not the spelling, punctuation, and formatting is correct. An editorially flawless manuscript is more likely to get picked up by a publisher than a flawed one, because a flawless one means that the publisher doesn’t have to spend as much money upfront to publish it. And if you’re a self-publishing author, an editorially flawless manuscript means that your book’s star rating on Amazon, B&N, iBooks, etc. won’t be dragged down because of readers’ complaints about typos.