1) Nope! Doesn't hurt! I've asked this question of multiple members of the admissions committee and they don't favor applicants purely because they checked that box. That being said, the school does have a mission of educating primary care physicians to serve Oklahoma. If you're passionate about staying in Oklahoma or practicing rural medicine it definitely won't hurt you to talk about that in your application! However, if you check the box and you aren't interested in rural medicine you could really shoot yourself in the foot if they ask you some specific questions about it and you sound like you haven't thoroughly considered your interest. To my understanding checking that box is kind of like your admission into the RMT since I'm a first year student and they haven't asked us anything about it since then.
2) The question is totally optional and I have several friends who left it blank and are in my medical school class. If you want to, don't be afraid to put literally anything in that essay box. I wrote about something pretty unrelated to my application, and they actually appreciated it and asked me about it in my interview.
Neither are required to be accepted, and I'd argue neither really increase your chances of acceptance
Best of luck and let me know if you have any other questions!