How are all those data points noise? They're crucial information used for targeting ads to a specific audience. Advertisers pay extra for it, because it leads to more sales. This is not just a gimmick, but a proven tactic that has made the web the most lucrative ad platform over any other. Adtech wouldn't be the behemoth it is without targeting, and the companies that do this well are some of the richest on the planet.
Take this simplified example. Say that you want to predict whether a driver will cause a car accident. You could run the stats and say that poorer, older, less educated, alcohol-impeded, sleep deprived drivers statistically cause more crashes, and then take an 80-year-old high school graduate with an income of $20K/year and say "He's three of those five categories, that makes his risk higher." Or you could observe footage of every minute of him driving, count the number of times he strays out of lane, turns without his blinker, doesn't look at the road, speeds, runs a red light, etc. Which is going to give you a more accurate picture?
Marketers build up demographic profiles because historically, that's all the information they have had available to them. The detailed record of everything their customer has ever done has been impossible to collect, or illegal for privacy reasons. Big Tech has that record. And they can use it to make much more accurate machine predictions about what a person will do than demographics alone can predict.