The reason there is .php is because it's faster and takes less bandwidth.
The .php extension is only to open cache, therefore there isn't any other file except ultimatebb.php (and vars_config.inc.php, ofcourse). If the cached file won't exist, it will redirect to ultimatebb.cgi and then it will be slower (both ultimatebb.php and ultimatebb.cgi instead of only ultimatebb.cgi), but it's worth it
I suggest you to use it unless you turned cache off or you're using hacks that don't support PHP, like WOL CGI version and jordo's PM hack, and it's important for you that they'll work every time. (jordo's version, for example, doesn't have a big problem because the forum summary should be linked to .cgi because .php doesn't support cookies, so in rare cases, that it's .php, it's not so important)