>So it's not so much a rehearsal of the dinner as a dinner in honour of the (ceremony) rehearsal?

Yep.


>Still strikes me as a huge expense in what is already a hugely expensive business. By all means get the guests a fish & chips supper, or a BBQ even ... :-)

Well, the people who are participating are typically doing so at some expense (clothing, possibly travel), so it's a way to say thank you. I've never been to a fancy catered rehearsal dinner, but most of the weddings in my family have had some kind of event the night before.

Tamar