Glad you don't have an understanding of how a website works. Websites are often loaded from a multitude of different domains. Each ad is often on a different service, using their own domain.
Pages load in a specific order, one query at a time, and your browser sits and waits for things to load. Say you've got 100 different domains serving content... That 16ms is now 1,600ms. I'd say that's a big difference, wouldn't you?
This is the second biggest reason people run pihole on their home networks, besides ad blocking. If you can bring the DNS server into your network, the "snappiness" of your Internet connection increases massively.
And no, this will not increase your upload or download speed. You didn't mention it, but other commenters did.
