I imagine Pihole is a just an example to illustrate a point. Once you have access to all network/http traffic, you can do an amazing amount of harm to ad-networks' ability to track and send you unwanted content. Imagine that kind of full access under the user's control. Not only that but you can cache/save/transform/manipulate how you receive the content you request on your device.

That's why the conspiracy-theorist in me thinks the tech companies have been pushing for HTTPs so heavily this past decade. It's "privacy" enabling sure, but at the same time makes it next to impossible to stop ad content and to break up tech data silos.