Having your friend, cousin, or partner disguise themselves as an agent to submit your work. It’s very easy for an editor to learn that Eldred “Slick” Bitsko isn’t an agent--especially when he has no other clients, let alone a track record. To some editors, this might imply that the trappings of publishing (“my agent”) are more important to you than the quality of your work.
Explaining in your cover letter how your last novel really was great, but “my publisher, Simter & Schuson, screwed up the marketing, and that disappointed me.” The editor isn’t trying to decide whether to buy your last novel. This is the sort of thing you might discuss with an editor once you’ve established a working relationship, but it adds nothing to the current submission.
Your name is Beldon of Atvar, but when the editor pops up the Document Properties dialog box in Word to get a word count (because you didn’t provide it), the name in the Author field is “Becky Lee Treversole” (your old girlfriend, who let you copy MS Office). Or maybe “US Robots & Mechanical Men” (your employer, who has no idea that you’re writing a novel on company time).