Even if you're not explicitly asked to submit one, you should "err on the side of sending one," says Glassdoor community expert MaryJo Fitzgerald. "It gives you a voice beyond bullet points on a resume, helps to introduce you to the hiring manager, and sets you apart from other candidates who may only be sending a resume or cover letters that aren't as good as yours." That's particularly important if you're applying for a job blind, without an "in" — i.e., someone to speak up on your behalf and fast-track you to the coveted interview stage.