[cellarius]

Because it's filled in your PHP script, but you never registered it for use in your template, and that's why it's empty there.

The tutorial does state (in bold red ) that you have to register every variable and array you want to use in your custom template. Try:

if ($post['field5'])
{      
     $templater = vB_Template::Create('postbit_name'); 
     $templater->register('post', $post)
     $template_hook['postbit_userinfo_right_after_posts'] .= $templater->render();  
}

Then you should be able to use {vb:raw post.field5} in your template.