Since the message of the apparitions that have been approved for private devotion are very consistent with their message, which is; repent and return to Christ for a great chastisement is coming; I would presume the purpose was to make the faithful take heed of their sins and to get right with God.
Of course, I will differ from you, lastchance, though hopefully amicably.
I did a study of Marian visions and messages. Granted, it has been a few years ago, and I am rusty on the subject, but I found way too much "consecrate to ME", and "MY immaculate heart" going on in it all... ME, ME, ME, MY, MY, MY... I know that is a generality, but it is the impression I was left with. I realize there is a message, as you define above, but it is secondary to the subject at hand, which is the apparition herself.
I find that to be disconcerting. YHWH's messengers (angels do this work generally) do not focus anything on themselves. the focus is always toward YHWH, to include the prophets and Christ Himself.
Or prepare them for the coming anti Christ who will convince them that he is the Christ.