Maybe I'm overstating the importance of these things by themselves. But when you're going against someone equally qualified, the manager has to differentiate you somehow. You can't control what makes a difference in their eyes. Maybe it's whether you make eye contact, or ask good questions, or dress better, or you included a cover letter (and that manager happens to like them). You can't control which of those things is the difference maker, so what's the risk in doing them? But the risk in not doing them is that someone else may look better than you and get the job.