Well, i have two teenagers at home that share the guest wifi password... I don't know what are these devices, i don't control them, but still, i want to control what i can 
I saw at least one of them using another DNS then the one that is given by my DHCP server.
With the iptables rules mentionned, I can force the dns to be mine and control some stuff...
In my case, pi-hole isn't only to reduce ads, but also, a good way to trap things that shouldn't happen on my network from guests in my home 
If "https-everywhere" wasn't a good thing that needs to happen, Squid would be installed on my Pi-hole server to also logs web request.
The teenagers in my home and their friends still doesn't know vpn... for now