The only functionality I can see it for is how it's used in tobacco shops. When you come in, you ask for the product, and they set the pack inside the dish. I think it's supposed to be kind of a neutral space. I've never seen anyone do it, but I think that you're supposed to put your money there. The dish might function as some kind of neutral transaction space, like how a handshake is supposed to show good faith. This way you can see exactly what you're buying, and the seller can see that the money's all there. My theory runs into a hitch, in that most people just hand the money directly to the seller.
Is it just a remnant of a bygone era that people don't use anymore, or is it supposed to still be a neutral space but just for the buyer? Maybe it's more about French hypochondria. We won't mention to the Francophones that the second you touch the money the transfer of germs is complete, and there could be something equally nasty in the dish, furthermore not counting the fact that French people also believe that you can get a cold by going around without a coat (seriously people, it's 2012. This might be our last year on Earth, but you cannot get a virus unless you come into contact with it.)
My question to you, chers amis internationaux, is this something that exists all over Europe, or is this just a Franquisim? I've never seen this in the US, and I did hear that actually in Japan you're expected to present the money for it before you receive the item. I just really have no idea.
New Years went well, and it's so far been pretty calm. Of course, we're also only three days into it. So well, who knows. In any case, best wishes for the New Year!