It's not just more insight on your personal preferences. It's the 100% certainty that one user on two different platforms is the same person. It enables companies to trade and sell your data behind the scenes with much higher levels of reliability. For instance imagine there was another TangoTrotFox on Reddit or whatever, and AI parsing of his statements led to a strong overlap in self identifying statements, vernacular, etc. You still couldn't be 100% sure it was the same person and so collating data would not necessarily be as valuable for advertising companies as it might otherwise be.

And yeah, I completely agree that this is a spot where the user "wins." Congrats, you get spammed with even more heavily targeted ads, political engineering, and more. What happy days.