In his answer of Details an applicant should include/exclude in an introductory letter to a prospective grad school adviser?, aeismail says
If the program in question is in the US, for instance, you should probably never contact the professor directly until after you've been admitted into the program in question. Since admissions decisions are handled centrally, it's just a waste of time.
Now I'm surprised with this, and I think I'm not the only one. For many times, not only in this site, but also in real-life examples in my university, I have seen that my friends get accepted through contacting professors beforehand, and those professors are helpful and willing to get you through the adcom. Recall it back, they don't go to US, but I don't think US should be an exception.
Why is it an exception? Why do the "admissions decisions are handled centrally", unlike other universities in the world, where the weight of the professors is heavier? Isn't that sorting the applications by score not good as checking their ability directly through interview? And who is the most suitable interviewer, if not the one who will advise you in the future?