It prevents the callback being called multiple times.  For example:

```javascript
cons.temp.render = function (str, opts, cb) {
  try {
    cb(new Error('this never goes well');
  } catch (ex) {
    cb(ex);
  }
};

cons.temp.render('', {}, function (err, res) {
  console.log('There was an error');
  throw err;
});
```

In there the console.log will be hit twice:

```
There was an error
There was an error

Error: this never goes well
  Stack Trace
  Stack Trace
  Stack Trace
  Stack Trace
```

This kind of bug exists in reality in lots of places in the code and it's really hard to debug, so it's easiest to just fix it at the top level.