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That service is free, this is not. Often times the student proofreading your paper has already taken the class and off much more guidance to what that particular teacher/professor is looking for. An anonymous web service, while convenient, doesn't have that added benefit.

How is a college writing center free?

Campus writing centers provide tutoring and feedback. They don't just fix it for you.

Well for one you don't have to be in college.

Clearly, I meant for students in college, which was the subtopic of discussion.