Here, I will mention just three of the many reasons why you should be contacting a prospective graduate supervisor before you apply to their graduate program. (I will get to some of the others, later).
First and foremost, you need to find out whether this person is even interested in taking any new graduate students. Most faculty members go through periods, from time to time, when they simply are not in the market for a new student. Reasons abound. They might have already made a commitment to another student for next year. They might already have as many graduate students as they can effectively manage and supervise at one time. Perhaps they will be out of town on a sabbatical leave for all or most of next year. Maybe retirement is looming within the next few years and its time to start downsizing. A loss of research funding, pending litigation, terminal illness… Like I said, reasons abound. And if you don’t find out whether this person is open to taking a new graduate student next year, you might end up wasting a lot of time, money, and hope on a fruitless application.
Another important reasons to contact a prospective supervisor before applying is to get some kind of impression of what he or she is like as a person. Remember that the faculty members in any graduate program are all different individuals. Different people deal with their students in different ways. A graduate student/supervisor relationship with each one of them would be different. You want to spend the next few years working with someone you like. On the other hand, you don’t want to work with someone just because you like them and you think that the two of you would be good friends. But, interpersonal compatibility is very, very important. Strangely, many graduate school applicants don’t give this very much consideration, probably because they fail to realize just how symbiotic the relationship is between graduate students and their supervisors. Many promising students end up with a supervisor who is a jerk. Many of those unfortunate students will quit graduate school before finishing, not because they are incapable of earning their Masters or Ph.D., but because working with their supervisor becomes intolerable for one reason or another.
A third reason for making contact before applying is simply to make yourself standout from the crowd a little. Most other students who apply to work with the same person will simply send the required application materials to the program. They will not personally contact this person, or do anything else to make themselves stand out. They will be relying only on how they look on paper. You will be far ahead of them by giving your prospective supervisor a reason to remember you, before they even get to see your application file. This can happen automatically when you pick up the phone and call, because it is so rare for applicants to have the good judgment to do so. You might be surprised to know how frequently it occurs that a faculty member implicitly accepts a new graduate student before he or she even sends in the application materials!
For more on how to choose an appropriate supervisor for graduate studies, check out this article I wrote for the MyGraduateSchool.com web site this past summer: http://mygraduateschool.com/ChoosingGradAdvisor.htm