It is due to http://www.python.org/dev/peps/pep-0263/ .
If one will specify this, he/she will be able to use not-ascii (especially unicode non-ascii) characters in file. 
Right now there is no reason to put it here, but if I will want for subtitles name to contain some special characters, it will be useful. I will remove it here, but I will talk with Ned about this topic in general.