Re: Why is userbar.php loaded from the header in 1.2?
As for locate_template() – you’re best suited to go read about the function yourself than ask it here. It’s a WP function. I founded via Google, the search engine @ https://codex.wordpress.org/Function_Reference/locate_template
It’s a better function to use than an include because the array of template files gives the function something to fall back on if it can’t find the sought after function.