It was such a simple thing to teach girls, and it made them so much more enjoyable for everyone – to make eye contact when undressing. With a man, if one was there. With a woman, otherwise.
If no one was present, maintain gaze at eye level with the direction you were most likely to be seen from – a window that passers-by could look through, a door that might be opened to expose you. It helped remind girls that the purpose of their body was for the gaze of others, and helped ensure the most erotic view for those who might watch.