Agreed with all the above. Yes, some reviewers might just glance over them and not fret too much if they are just "OK," and even fantastic essay materials are unlikely to get you interviews at schools that are super high reaches. However, they can certainly make a massive difference if your application is borderline, and they can always ruin any otherwise good application.
Additionally, for me personally, I had kind of a unique/personal background story in my PS. I had at least one interviewer at almost every school have a discussion with me about it or very kindly comment/compliment something I had said. I also had some interviewers read specific quotes of mine from my activities section or my essays back to me -- so as Dr. Fencer mentions, it's definitely a good idea to remind yourself of what you have said before you interview.
My motto: if you're at the application stage, you have undoubtedly put in YEARS of hard work to set yourself up for this point when it comes to research, volunteering, leadership, grades, etc. You are SO close, and it is not worth it to take a chance on anything!! Apply early, apply smart, re-read everything... you don't want to put yourself at a disadvantage just based on the way you've applied. Maximize your chances to show off everything you've worked for over the years!
