That's already built into HTTP with the Referer header. So what's the point?

Some people might bookmark the link, copy it, view it later, share it, etc.

It's helpful to know that the original source of the link is producthunt (in this example).

Sometimes links will go through a number of other things (url shortener, analytics, so on) before hitting the final URL.

I think the point was that all these clicks from HN are probably misleading the analytics provider.

(Assuming the site is integrated with an analytics provider that uses that query parameter to determine the referrer.)

That is correct. This was the intention.