To be honest, since I have adblock I find it hard to understand too. How can that make so much money? But all the current richest companies and people built their empire on private data right? Google, Facebook, elon musk and so on, so it's very easy to see the proof. You give them your data, you give them money.
Now, power is easier to understand. If you want to control or kill people, the more information you have the better. It's easy to see why evil governments have always loved spying.