Sure, let me connect the dots for you.
1. Let's say there are two parties: pink and purple.
2. You absolutely love Pink Party because they want to paint the roads pink and force everyone to drive pink cars.
3. I'm running for Purple Party, which says everyone should be allowed to drive whatever color car they want, but we also want pink roads.
4. Gerrymandering is illegal, but our team determined that we can sway some areas. You live in one of those areas.
5. My team writes up an ad saying something like, "Vote Purple and support pink roads and pink cars!" We target all the Pink People living in the aforementioned areas.
6. Pink party runs a very similar ad.
7. You get confused, wasn't there one of them that didn't support pink cars?
8. You google the parties, and visit our website to read our platform. We see that you were in the ad segment, so we hide our support of any color car and change it to say that we're for pink cars.
9. You give up, and just decide that you like my beautiful face -- but more likely you just didn't see the difference, so you vote Purple Party at the election.
10. I get elected and we get pink roads. However, the cars are quite multi-colored.
Hopefully, you can see how this is dangerous? It's quite different than without targeting (where everyone sees the same message). The ads I saw told me what I wanted to see, while the ads you saw were what you wanted to see and confused you enough to vote against your own party.