Apparently it's a thing for the past few years -- you set up the typewriter at the wedding reception, and the guests get to type warm loving wishes for the couple's happiness, or (as would probably happen in my family) silly stories about something the groom did when he was three years old. Or whatever. Then all the pages get put into a binder and stored with the other wedding things. I was at Cambridge Typewriter a few months ago and Tom was selling a shiny Corona flattop to a couple for their wedding. He had to demonstrate pretty much everything since neither of them had used one before. 

Thanks for the info.

But why not write the messages in a book, I wonder?  Would this be because so many people have terrible handwriting these days? - or perhaps it's just the novelty.

If it keeps a typrwriter, and a typewriter shop, in business, great!

I just remembered I made my wedding invitations on the typewriter, I'll try to scan one and post it here.  To be honest, I just typed the one, then photocopied it onto nice card and cut them out by hand.  I couldn't guarantee typing out 200 invitations without making any mistakes...