When I copy a link to share a tweet, or a YouTube video or any number of websites, there's often an ID unique to me attached to the link. (YouTube even changes it for every share)

e.g. https://youtu.be/dQw4w9WgXcQ?si=2W5B4wtJZFr7-Ps4 (yes, it's Rick Astley)

I understand it's for tracking and analytics generally speaking, but practically, what do these sites really get from them?

The signals from me sharing a particular link, and others clicking it seem very minute and not very valuable. I suppose you could establish parts of a social network over time, but how valuable is that in practical terms? What can they do with that, that they couldn't do without? How is it worth it to add to all links and have the infrastructure to store and track and analyze?

Is it simply cheap enough that "why not"?

Thanks!