All this did was move the previous
```javascript
   conn.query(sql, values, function (err, rows, fields) {
     conn.release();
     cb.apply(this, arguments);
   });
```

The purpose is to cal the callback with the same `this` that `conn.query` would have called it with such that `pool.query` and `conn.query` have the same `this` instead of possible different ones. This allows people to use `conn.query` and `pool.query` interchangeably.

If you were wondering what `this` is pointing it, it is point to the `connection`.