The difference with pi-hole is that since it blocks it at the DNS level the network traffic doesn't even reach you, saving on bandwidth.

It will also be faster than most browser based ad blockers since the browser has to analyze every webpage whereas pi-hole blocks by simply not making a request.

Also if you want a DNS that works outside your local network it would be trivial to get a cloud server for $5 a month put pi hole on it and route DNS through that. Then you could get free https certs and have a pretty sweet private DNS just for you!

And if you're someone trying to setup their own local DNS it's likely you have a spare pi or computer laying around somewhere to use.

And finally it offers pretty great convience. Instead of having to setup blockers on every device it just needs to be set on the pi.