They need to track so that they can "prove" they did what they sold.
The challenge with having access to that much information is that it can be very powerful and a very lopsided/asymmetric power. I remember when Facebook had Open Graph Search-when any user on the platform could perform complex searches of their friends, friends of friends, and more. It was so powerful at times it freaked me out how powerful it was. I could search for friends of friends who are women who live in San Francisco who speak Spanish who are single and who like Dave Matthews and it would give me a list. Facebook shut it down a few years ago, but I imagine something similar still exists on the advertising side of things (side note, there was someone a few years ago that used the advertiser tools to send ads specifically to his one friend as a prank). So for me, why should some groups have access to this information and others not? Who gets to control who has so much info?
Again, the info itself is not necessarily the problem I see, it's the asymmetry and how those people use the asymmetric info they have on me, which can be to powerfully help me or hurt me or maybe even ignore me.
I don't agree about the good intentions. The tracking is a means to an end. People are tracked to measure engagement and to target ads, both of which have the purpose of increasing the value of the advertising to the benefit of the ad networks. They don't care if you get relevant ads. They care that they can charge more for your eyeballs because they can classify you as "male, 19-29, in USA, interested in technology, art, exercise" and sell you as part of a package to companies wanting to target that market.