That and you can generally guess from the post mark where it's come from.
Main idea really is that it's secret, a random nice gift from someone who haven't a clue who it is.... (lol unless they give it all away.. some people can't keep secrets to save 'emselves

)
The other main idea is that its a gift, for you, because we tend to think of ourselves last, not necessarily having to be perfect, expensive, the best gift outta the whole lot, but a nice thought. That and it's a good way to get to know other people, even just a tiny bit of info.. if we all got people we knew well, it wouldn't be that fun.