They get as much information from you as they possibly can. Age, sex, gender, weight, phone numbers, addresses, work history, purchase history, income, tax records, net worth, family and friends, hobbies, kinks, criminal records, food preferences, medical history, etc.

All of it is worth money, because the more data they have about you, the better they can predict what you will be interested in buying. They want to target ads towards you that have the highest chance of getting you to watch/click them.

Even if you think you aren't influenced by advertisements and marketing, you are. And remember, it isn't just you, they will use your data to target your friends and family. So even if you don't care about all your personal data being mined for targeted ads, you should care about your friends and family.

Also, they more data these government agencies and corporations have on you, then worse it is when (not if) they get hacked. So even if you have no issue with these companies and government agencies storing your data, you wouldn't want hackers and scammers to get that info and use it to hack your accounts and the accounts potentially of your family and friends too.

It's about having control over your information and keeping yourself safe and protected. If you truly had nothing to hide, you would walk around naked, leave your door unlocked with your valuables inside, hand random strangers your credit card number, and leave the bathroom stall open while pooping. But most likely, you don't do any of those things when you're out and about, and the same should be true for your digital life.