FreddieFox, this is what I posted on another thread, because it's a reasonable summary. BTW, I can't tell you whether to download this app or not! You may decide that, on balance, there's a benefit that's worth it.
The more data an organisation has on you, the better it can profile you, know your want, needs and emotional reactions, the better to manipulate you.
For example, this is what Cambridge Analytica did with data.
(CA happen to have been hired by the pro-Brexit campaign and Trump campaign – but they could as easily have been hired by the opposition. And anyway, other organisations can do the same tricks.)
Cambridge Analytica: how did it turn clicks into votes?
www.theguardian.com/news/2018/may/06/cambridge-analytica-how-turn-clicks-into-votes-christopher-wylie
Cambridge Analytica could, Wylie says, craft adverts no one else could: a neurotic, extroverted and agreeable Democrat could be targeted with a radically different message than an emotionally stable, introverted, intellectual one, each designed to suppress their voting intention – even if the same messages, swapped around, would have the opposite effect .
Wylie brings up the anodyne political statement that a candidate is in favour of jobs. “Jobs in the economy is a good example because it’s a meaningless message. Everyone’s pro-jobs in the economy. So in that sense, using just the message of ‘I am in favour of jobs in the economy’, or ‘I have a plan to fix jobs in the economy’, you cannot differentiate yourself from your opponent.
“But one of the things that we found was that actually when you unpack what is a job for different people, different people engage with constructs with different motivations and value sets that are interrelated with their dispositions.”
What that means in practice is that the same blandishment can be dressed up in different language for different personalities, creating the impression of a candidate who connects with voters on an emotional level. “If you’re talking to a conscientious person” – one who ranks highly on the C part of the Ocean model – “you talk about the opportunity to succeed and the responsibility that a job gives you. If it’s an open person, you talk about the opportunity to grow as a person. Talk to a neurotic person, and you emphasise the security that it gives to my family.”
Thanks to the networked nature of modern campaigning, in theory all these messages can be delivered simultaneously to different groups.