[ImaCake]

Mostly its because of what they (as in advertisers and platforms that display ads and take data) can do with that data once they have it. They can figure out if you're moving house, suddenly interested in a new hobby, pregnant, or emotionally unstable and thus more vulnerable to suggestion. Once they have statistically guessed at these possibilities, "advertisers" can target you through social media platforms and google ads. They can try and nudge you at just the right moment, many people will be unaffected, but the advertiser hopes enough will be nudged into buying something, voting differently, or otherwise changing behaviour to get good bang for their buck.