Or controlling crowds of protesters by inducing pain.
Taking it up a notch, reading emotional states could help figure out the general mood of the population, as well as specific subsets or individuals, and either figure out how to make potential dissenters happy (à la Brave New World), or get rid of unsatisfied members of society (1984).
Finally, I think (not sure) I remember Musk talking about one day storing and retrieving memories via such devices. This would allow to pretty much rewrite individual and collective history. Most definitely way beyond our current technology’s capabilities, but way better than whatever Big Brother could ever dream of.
Humans in capitalist societies will be split into two "species", plain vanilla lame brains like you and me, and the augmented super-geniuses who've augmented their brains with the sum total of all recorded human knowledge.