In almost all cases, $CFG->httpswwwroot is the same as $CFG->wwwroot, which is common.
The only place where they are different is when you have the 'longinhttps' setting turned on. That is, the site uses https for login, but nothing else. (In which case wwwroot will be http, but httpswwwroot will be https.) This is no longer recommended. If you care, you should be using https for everything these days.