Does it write to a file?

stylus.js

var express = require('express')
    , css = require('stylus')
    , str = require('fs').readFileSync(__dirname + '/public/css/style.styl', 'utf8');

// What makes sense to me is pass in "str" to be rendered and write that data to the file named "style.css". 
// But examples show it being named "style.styl" which I thought was already loaded from "str"???
css.render(str, {filename: __dirname + '/public/css/style.css'}, function(err, css){
  if (err) throw err;
  console.log(css);
});