I went and disabled personalized advertising, and for the first time in years I started seeing ads that tempted me to click through, ads from tech companies about tech. Why the difference? Because YouTube started showing me ads related to the content I'm actually watching rather than based on their faulty neural network profile of me.
Obviously this is just one anecdote, but it's illustrative of the point: don't try to advertise either beer or iPhone covers on a car maintenance site. If I'm on a car maintenance site, you know all you need to know about where my interests are at that exact moment. Why try to hijack my attention (difficult when I'm trying to solve a car problem) when you could instead subtly redirect my attention to an ad for a local mechanic?
If there's only space for one ad, do they show you an ad for tools, exterior mechanic, engine mechanic, or a new car ad? All are valid options for both you and the advertiser.
This is where invasively tracking your online behavior can help you.
Edit: ok endless is hyperbole, but content targeting is hardly useless.