Honestly it probably only helps installation costs, and not by much. There are no code issues with running standard direct bury copper in the same trench as direct bury fiber, nor are there any code issues (iirc, I am not an electrician) with running them in the same conduit even. Plain direct bury OS2 and 12/2 or 12/3 copper is dirt cheap and the fancy cable they use for generators that has 12/3 plus additional current sense conductors isn't that much more expensive.