They're usually crap and get left on the table in my experience.
I'm planning on buying scratchcards or lottery tickets for them.
Probably another American thing that has crept into our way of life to make couples spend more money.
This is something I assumed as well. I first heard about them a couple of years ago, it seems totally stupid to me. We certainly never had such tosh at our wedding and I don't remember any wedding that I've been to more than five years ago where such things happend.