Colleges provide "writing centers" where people (presumably upperclassmen in a relevant degree program) offer proofreading. I don't see how this is much different.
Campus writing centers provide tutoring and feedback. They don't just fix it for you.
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?
Well for one you don't have to be in college.
Clearly, I meant for students in college, which was the subtopic of discussion.