In theory you can track individual devices in a household instead of grouping everyone in one house, small business, building, etc. as one IP.

Currently, even with the limitations of IPv4, most trackers can use cookies and browser fingerprints (your browser's headers, fonts, etc.) to individually identify you even behind NAT. But with IPv6, ad engines could potentially identify that you're running two different browsers on the same laptop or device, and associate both of those browsers to _you_ (some type of single targeting profile).

They also have a more precise way to track you without a browser fingerprint.