[Edwin]

It's not optional - it's a link word that is used in certain contexts, like "of". There are some expressions that will make no sense at all without it, even though Overture strips it out. Japanese speakers automatically know when to use it or not, it's only if you're grappling in the dark armed only with Overture, a dictionary and Google that it seems confusing.

Please listen to the folks who speak the language!