It is not, strictly speaking, “an implementation detail.” When you are dealing with external data collections (be they SQL tables, or XML files or whatever), the notion of “addresses” (hence: references) does not exist. The notion of “keys,” of whatever format you wish, does.

If you have ever had the unfortunate experience of dealing with an IMAGE or an IDMS database in any past-life you'd much rather forget, then you will know exactly what I am talking about...   :-D

(You do not, of course, have to answer that. Many I.S. memories are much better left buried in the past.)