Targeted advertising lets you run a campaign that never could be run before... one where you appear to be something different to different people. If you were trying to seize control of three different warring groups, you could advertise to A, "We'll kill B and C!" , to B, "We'll kill A and C!" and to C, "We'll kill A and B!" which you couldn't do in a stump speech without people figuring out the ruse.
By building a detailed psychological profile of individuals, you can build a model that allows you to tie their responses to these questions with the political messages they're susceptible to. Cambridge Analytica paid a few hundred thousand people to do a quiz where they shared their Facebook likes and answered questions about their personality. CA then used that to build a model that showed "People who live in Slidell, Louisiana and like Dodge Ram trucks will be most receptive to messages about illegal immigration and are generally supportive of state violence". Then they can run that ad to everyone in Slidell who likes Dodge Ram pickups.