I'm working with a couple of aspiring writers, editing their first attempts at young adult fiction. Both are thoughtful, bright, clever people. Both have teaching experience and/or kids at the grade levels their work is aiming toward. Both would be fine dinner party guests; both could carry a conversation with wit and style. However, both have a ways to go in their writing, including understanding one key, technical matter: manuscript format.
It's a cliche' that you only have one chance to make a first impression. But all cliches are grounded in truth. The first impression you make as a writer is your manuscript-- a visual impression. With people, we judge them first on appearance, on how well "put together" they are. We use basic criteria of clothing, tidiness and all the little markers of personal style. We do this without fail before we get to know them, that is, understand their personality, their values, their world view.
Recently I was struck by a photo of people lined up for a jobs fair--how consistently tidy and well dressed they all were. They had researched "the look" of a job applicant, and conformed to it. Likewise there are formal rules for manuscripts, and editors can see immediately if you know them. Most publishing companies use "MLA standards", meaning the Modern Language Association's guideline for manuscripts. The formatting guide is boring stuff, but every professional writer uses it (or something similar for scholarly writing). MLA formatting speaks to margins, paragraphing, line spacing, pagination, etc. In general, your manuscript needs to be double-spaced with one inch margins; have paragraph indents (not block paragraphs, which are for business letters); and your name and the page number on every page. My two aspiring writers had somehow missed all of that.
Think of your manuscript like those people in the job fair line. You don't want to be the one wearing jeans and tennies when everyone else is wearing a suit. If your manuscript doesn't mostly conform to common standards, the editor begins to read it with great skepticism. And that's the last thing you want.
Postscript: I have also encountered aspiring writers who obsess over format at the expense of the writing. Their novel becomes an exercise in secretarial exactness! Put content first, obviously, but make sure it's at least close to the common standards of publishing (double spacing always!). The goal is keep your manuscript moving forward. As one editor told me, "I'll read a manuscript until the writer gives me an excuse to stop." Don't let appearance be that reason.