In many cases, you just want to interpret a script and continue. The script should print any errors and then you are ready to do something else. That's what this flag means. In other cases, you want whatever the call returns returned to you, even if it was an error. For instance, you may want to pass the result along, error or otherwise. In this case, don't include this flag and the return value of the called function will also be copied to your own return value.