What is 'currentHost' for, in defaults?
In the case of networked accounts and networked home folders, a user can login to any Mac on for example a company network and see their own files and have their own preferences set.

This is the case where you can use the -host feature. Say that you want for example one wallpaper on your own desktop, but another when you login to the shared office Mac in the meeting room. Or any other type of setting than wall paper ofcourse.