what is the difference between render() and drupal_render()? what is the difference between render() and drupal_render() anyway ?
Is there a reason to use only drupal_render()?
The docs for render() explain it pretty well:


  This function renders an element using drupal_render(). The top level element is shown with show() before rendering, so it will always be rendered even if hide() had been previously used on it.


render() is basically just a wrapper for drupal_render(), except that it makes sure the element passed in is set to be shown when passed through to drupal_render().

It also makes sure the element passed in is an array (which is all drupal_render() can accept). If it isn't, the element is return back as-is. So it provides a bit of a safety net if you like.

If for any reason you have a variable you need to render, and you don't know if it's a string or a render array, you can pass it straight through render() without having to test the variable type yourself.

For some real-world examples of that sort of situation there's a list of calls to render() you can look through.