This is to prevent bubbles, extreme polarization, and psychological manipulation. The goal is to prevent another Cambridge Analytica.

Also if you think about it, it would be bad if different people saw completely different political ads. Party could "promise" one thing to some people, and completely contradictory thing to other people. Since people would live in their tight bubbles, most of them wouldn't notice. And this ("dark ads") is something that is definitely technically possible right now.