Say you have 3 devices MacBook, iPhone & iPad. You start storing all your original photos on the MacBook, as you want this to be the centralised location. iPad & iPhone are configured for "Optimised Storage".
As soon as you start shooting pictures with your phone, originals will be stored on phone and MacBook. Or images are uploaded from phone full quality and then downloaded to MacBook as unmodified originals and left on phone as smaller miniatures?