Educating medical students is expensive-- it costs time and a number of resources. When students fail to meet the demands of medical school and drop/flunk out, the school's investment has essentially gone to waste. The MCAT is a standardized test that allows admissions officers to quantitatively compare applicants to each other and determine which applicants are most likely capable of meeting the demands of medical education.
Tens of thousands of premeds apply to medical school each year. Many of these applicants score well on the MCAT. Generally speaking, a good score will not get you into medical school, but a bad score will be more likely keep you out. Sometimes, it is impossible for every single application to be read from start to finish. As such, many schools use the MCAT as a screening tool. Applicants with scores above a certain number will be considered and those below the cut off will not. Different schools have different score cut offs, but the average score of successful applicants ( students admitted to at least 1 school) hovers around a 30.
While the MCAT is an important component of the medical school application, it important to note that it is only one of many factors. In terms of gaining admission to medical school, a 45 may not make you any more "special" than a 35 because it is obvious that you are capable of doing the work. Once you have passed the initial screen the other parts of your application-- your personal statement, letters of recommendation, and interview-- are probably more likely to "get you noticed".
Medicine is a service profession. According to HMS commencement speaker, Dr. Donald Berwick, "we [get] our compass the day we decided to be healers. Our compass is one, simple question, and it will point us true north: How will it help the patient?” Those who enter the profession are committing themselves to a lifetime of helping others. In my opinion, a history of volunteerism is an important component of your application to medical school. How else are you going to demonstrate your commitment to helping patients unless you have given your time and used your talents to serve people in the past? ( I can't really speak for MD/PhD applicants, as their applications may call for different things).
You NEED clinical experience when applying to medical school. Many applicants gain clinical experience by volunteering at hospitals or staffing free clinics. That being said, working in a hospital is not the only acceptable form of community service. There are many ways to spend your time helping others in need. These service activities can be clinically focused, but that does not always have to be the case. Make sure that you are passionate about the causes to which you dedicate your time, especially if you write about them in your applications. You you will definitely be asked about these activities on the interview trail. If you choose non-clinical activities however, make sure you are able to get clinical experiences in as well. Shadowing a local physician is an easy way to learn more about the profession.
It's hard to say what will get you chosen for an interview at one school or another. Each school is really looking for something different and emphasize unique things when filtering a pool of applicants. My best advice is to find your passion and pursue it. The path to becoming a physician begins long before medical school matriculation. This path entirely too long to "fake it till you make it." Spend time doing what makes you happy and articulate that passion during every step of the application process, from your AMCAS to your interviews. Schools that appreciate your spirit and value what you have to offer will pick up on that!