It creates a clear distinction between user-defined fields and protocol fields. User-defined fields are only for display (and only for display in a confined region of the profile), while protocol fields can actually influence the behavior of the account, like set your Lightning address or avatar.

User-defined fields are an amorphous blob the user can do whatever they want with but should not be relied upon in any way, kind of like pronouns. 

 Right, so if pronouns are just listed as a field when the user visits a profile, it works. But if they are added after each person's name in the feed, then it becomes something else.  

 Yep.