Save the (pre) Date before the Save Date before the Invite!

I received a "Save the Date" card from Bald Gaz and his vict- uh, fiancee Tracee the other day. A very kind and generous act and yes, the date is saved (unless this entry gets me banned).

It got me thinking though: what an odd concept! "This isn't an invitation, mind you - just putting you on notice." When did these pre-invitations come into existence? I know when Erica and I Did the Deed in 1997, we'd never heard of the phenomenon. And what purpose do they serve? Isn't "saving the date" the whole point of invitations? Does the lack of an RSVP request somehow necessitate a whole extra layer of communication? Perhaps it's meant to give the recipient a good head start on making other plans? Or to give the happy couple one last chance to cull the list before sending out the "real" invitatons?

Will we someday find a "Keep an Eye Out for the Save The Date Card" announcements in our mailbox?

I remain increasingly puzzled by the culture in which I live.