Sell it to people who will manipulate you and your family. Even subtle changes could shape the direction of your life. Or, they could say people with your preferences are in a certain area and fund some other area instead. Could be anything. The less data they have, the better.
Did you not see the documentary on how the private advertising company used Facebook to run ads to specific demographics of people to influence an election?
Also your comment about trying to sell you stupid shit, you may think the ads are stupid and that you'd never buy the stuff or pay much attention to whatever idea its pushing, but maybe the idea of the advertisement isn't to instantly sell you on something but slowly change your thinking subconsciously?
I also like to think I'm impervious to a lot of marketing BS, but then again am I the same as the people who say, people are stupid for getting scammed, only to fall victim to a scammer because they thought they were smart enough not to?
The advertising company you mentioned is Cambridge Analytica, right? If so, and not something else, I'd like to add this shit has come to light and exploded in 2018. We have to wonder how many others went dark after that and what shit they have come up with since then, in these 5 years.
To be fair I do use the basic level of protection, ad blockers and what have you. So if they are trying to influence me with ads, well, I’m not seeing them.
But I don’t go the full 3 layers of ad block with a pihole and a VPN.
Big into drinking, lots of bars and going to liquor stores whilst your mobile phone is with you and some apps have location access permissions?
Don't be surprised if your health insurance goes up on account of being in a higher risk group...
(It's not exactly hard to cross your location with store locations and it's probably already done generally to try and determine your consumer habits)
And this is just a mild, mild example. Stuff to do with people's sex life can have far more entertaining effects, especially in a highly moralistic country (like the US) or if cheating on a spouse.
Think about it this way: if you don't jump through hops to make it hard to track your location, there is a record, forever of every place you go to with your phone and how long you stay there (and repeat visits with long stays would signal a pattern) as well as of everything you're interested in enough to look it up and/or visit it on the Internet and it's all crossed. Further, if you have email via one of the big providers such as Google, every email you sent, received or even just drafted but never sent is tracked.
Why do you think Google started pretty much forcing people to give them their phone number for the Google account that, for example, goes with their Google Mail?! It allows them to link all that sweet information and match it to a single individual.
And this is before we go into crossing data with the kind of physical life data on you: the insurance company records, car onership and rental, financial information and transactions, even public transport use (for those which use modern touch-in touch-out cards).
sell it to police. sell it to a stalker or vengeful ex. sell it to prospective employers, landlords, debt collectors, and scammers. this is just a taste, off the top of my head.
Right now in the US? Not much outside of try to sell you stupid shit.
But the capability in the data is there for an authoritarian regime to do quite a lot to you as a result of the information.
So you know - if there was any pending threat of a narcissistic psychopath upending the government you might be concerned over what insights into individual mindsets can be accomplished with the data available...