It is not about having your particular data. Is about the aggregated data of a population. On a shallow level that information can be used to engineer ads to appeal to a greater audience. On a deeper level, population dynamics can be used for nefarious purposes such as driving them to an agreement that wasn’t in their best interest. Brexit and the Trump election are two examples of this being possible thanks to data from Cambridge Analytica. See voting paradoxes and Asimov’s psychohistory that each day is getting farther from fictional.