What is the purpose of the website zuowen8.com, which contains student essays?
It's called 作文吧。


  What is the purpose of the website? E.g.:
  are the essays uploaded by students? Are they error-free?
  is this meant to be a student resource, or a system to prevent plagiarism in essays?


I think this website is for primary school/secondary school/high school students. The purpose is to provide some good examples to show how to write a good article/essay. 

It's a kind of student resource, and usually, make it easy to plagiarize. Not sure if it's bug-free. But it's "high-grade" in exam.


  What does the website's catchphrase (让写作成为一种习惯) mean? My guess is: Let writing become (something of) a habit. (is 一种 a quantifier here?)


It's a slogan. Yes It's "Let writing become a habit".