If you are on your phone showing something to some person, and you get an ad for a new pressure cooking. Well, there it is a nice pressure cooking.
Now if you have been looking for something else that you want to keep private (gay clubs, abortion clinics, or anything embarrassing) then your phone has betrayed you.
There is also a point that if the ad is more useless, the quantity of ads should decrease because advertiser will not find them worth it.
Worse, say a gay club near you was attacked by someone who made an explosive device using a pressure cooker. Your interest in those things (actual or as determined by advertisers) could cause you to be a suspect and/or arrested.
The problem with accepting being under constant surveillance to make advertisers money is that the data is never just used for ads and even if you never show your phone to another living soul that data never goes away and can end up in the hands of just about anybody.
Does not work as imagined by most, and is easily detected by sites. So basically just a convenient way to have your cookies and other site storage deleted. But (meanwhile) basically useless because of other ways of 'fingerprinting' client browsers.
Now if you have been looking for something else that you want to keep private (gay clubs, abortion clinics, or anything embarrassing) then your phone has betrayed you.
There is also a point that if the ad is more useless, the quantity of ads should decrease because advertiser will not find them worth it.