For targeting advertising expendatures at the site level. If most of my traffic, as revealed by referrer links, comes from social-media-platform-foo and only a little from social-media-platform-bar, then I am likely to spend more on ads from foo than from bar. I'll grant that it is a noisy measure, but doesn't need to be about tracking a particular individual.

Businesses survived just fine before this. Do personalised ads earn more money? Maybe. But they're invasive and if the governments bowed down to the people instead of corporations they'd be just as illegal as stalking a potential customer to harass them when they're most likely to see you