It's about someone "connecting the dots" between ALL your public activities.
Someone following you OUTSIDE their "space" that you visited and then invading your other spaces.
People don't usually want to mix their social circles. e.g. nobody likes a snoopy overprotective parent following their [teen] kids around wherever the children go with their friends.
Nobody likes their spouse coming uninvited on a night out with the guys/gals.
Or, when you reply to someone on HN or Reddit, and they don't like what you say about 1 specific topic, then they go and downvote ALL your comments about everything else you said!
That's perfectly legal. It's allowed. Nothing's stopping it. And hey, you commented publicly so you were "asking for it", right?
BUT IT'S A FUCKING DICK MOVE
I do understand what you are getting at. I just don’t find people repulsive by being curious of me; even if they find materials I myself did not make public.
As an aside, I sometimes stumble upon websites of people who write and show pictures of themselves and their family. I find them beautiful[0] to visit and hope more people would do that, as was more common 20 years ago.
If they do it just to reach out to you with good intentions then it's fine. But if they do it to dox you or harm you in any other way, then it's not fine.
Disturbed people with access to this data can use it to harm people, like the Ebay exec who stalked a critic or the occasional police officer who stalks a romantic interest.
Customers of such datasets: banks, insurance companies, HR companies, marketing firms, governments, etc.