Because usually when they do that, they’re using the object to localize the string. So to you it says it in english, but if $this 's localization is spanish… it’d be in spanish…
Underscore is a common alias for gettext or localization in general. PHP’s gettext extension for examples defines “_()” as an alias to “gettext()” http://us.php.net/manual/en/function.gettext.php So when you do something like “_( 'Hello' )” if your localization is currently set to Japanese you will get the Japanese method/text of “Hello” instead of English.