Comment by hansvm

They can get a profile of when you're in which room, how many people are in the house, etc. That's useful for targeting (e.g., up at 5am, leaves, comes back in an hour or two will have you profiled (more complicated than this since it's usually a fuzzy vector thing, but for the sake of argument) as somebody who goes to the gym, maybe leading to creatine advertising or other whatever), and also for attribution (e.g., a TV ad is shown, somebody gets up, and a purchase is made from a device known to exist at that household, regardless of whether it's behind a VPN).

If you have even very crude data from somewhere else for the targeting, improvements in attribution tech are actually the more important factor. The adtech company mostly doesn't even care who you are, just whether the ad turned into a purchase or not, and that's where a lot of the invasive tracking comes from. They'd be perfectly happy with a quickly changing "identity" if they knew it was reliable and stable between ad and purchase.