Let's say I'm a transit provider [like Google]. Right now, I see TLS traffic from your public IPv4 address to another public IPv4 address. I have no idea how many devices are behind those public IP addresses. I can maybe combine a few assumptions with my browser-based tracking, but it's difficult to identify you in apps I don't have control over.
Switch to IPv6, now I know exactly how many devices behind your firewall are talking and how many devices you're talking to.
Even better, I can combine this with a browser-based tracker and I can see what you're doing in apps I don't control. I first identify you with the browser and what IPv6 address you're using. Then as you use other apps that I can't spy on, I just have to look for your IPv6 address in my transit traffic to figure out what you're doing.