Think about it, Google Maps is (I think) the biggest collection of map data and information on locations everywhere around the world, yet I don’t want an advertising company knowing that I just went to a fast food place.
YouTube is an endless trove of video content that anyone can contribute to, yet I’d much rather not give an advertiser information on every little interest that I have in video.
It’s so bad for you, but it’s so good.
They can fingerprint your browser and (very) probably your hardware, along with ip, location, and other leaked data, allowing them to reliably identify you whether you’re logged in or not.
At this point, it’s more of a symbolic protest.