Sure, but they are for measuring the effectiveness of marketing channels, not simply the direct referrer. They can discover the direct referrer through HTTP headers. If a marketer promotes something on Product Hunt, then somebody on Product Hunt submits it here, then you follow the link from here, it means the marketing channel that reached you was Product Hunt. It gives marketers better information in aggregate about which marketing channels are most effective.

I’m glad to understand that, though it doesn’t make me any more inclined to add any extra metadata to my web traffic.