Fields on related fields, don't get populated when fetching the main entity.
They will be populated when the related entity get fetched.
What they are used for?
This gives you the ability to create a property on an entity, that get's it value from another property/field on a related entity, might be useful when databinding, if you don't want to access related entities at databinding.
So don't use them unless there is a need for them.