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".
