This goes for query letters to agents as well.
Also, that emphasis on the submission guidelines (or style sheet) and formatting things EXACTLY the way they requested it? Yeah, that's so that they know at a glance whether you have a brain in your head and can fucking read. Didn't follow the guidelines? They can discard your submission in an instant rather than wasting the two minutes it takes to read your cover letter.
FOLLOW THE SUBMISSION GUIDELINES!!!!! THIS IS STEP ONE OF "PROVING YOU'RE A PROFESSIONAL AKA SOMEONE WHO SHOULD BE PAID MONEY FOR THEIR WORK".
FOLLOW! THE! SUBMISSION! GUIDELINES!
Wait hang on there's still a couple of you who are not internalizing Follow The Submission Guidelines. I will tell you a story.
Couple years ago, I taught a college course on Writing & Publishing Scifi/Fantasy. Towards the end of the 8-week workshop, I told the class that they were going to learn what it is like to be a literary agent. I asked them to tell me a few things about what their dream novel would be if they were an agent (genre, themes, etc) and then I went and wrote fake a fake query letter for each of them. Then I scraped together a bunch of other query letters from Queryshark, and then I wrote some unhinged ones. Printed them all out, put them in a box, walked into class on the day, said "The first person to find their Dream Client in the slush pile wins Twenty Real Human Dollars." The air in the room suddenly became *FERAL*. RABID. College students will literally kill a man for $20. I dumped the box on the floor, screamed "GO!" and watched them throw themselves into it.
You know what happened? Almost instantaneously they developed a sense of "UGH FOLLOW THE GUIDELINES." They were ruthlessly throwing things aside simply because it did not include a "Dear [your name]," salutation. They were crying, "NO!" when they got a query letter for a short story instead of a novel. When confronted with a pile of garbage with a couple gems in it, they figured out in nanoseconds that the #1 red flag for garbage is "did not follow the submission guidelines."
FOLLOW THE GUIDELINES!!!!!!