I'd argue that the latter purpose---allowing the reader to recover the original sources and arguments to your thinking---is more important than the 'giving credit' one. If a conversation or email exchange with someone was indeed critical to the development of your ideas, then you offer them co-authorship, not a "citation' to something not only unpublished but undocumented. If it was only incidental to the thinking, then no citation is necessary.
I will note that many a graduate supervisor or lab administrator has been offered (or simply taken) co-authorship for a far more scant contribution than a key conversation.
