YouTube ads are notoriously tricky to block and can’t be blocked by any DNS-level ad blocker I have tried. If you don’t use YouTube then they probably work well for you, but a lot of people do use YouTube and want those ads gone
Not everyone has mullvad, corporate VPNs can’t be run concurrently with mullvad, not all networks allow a VPN, not all sites can be accessed through a VPN like mullvad, not all countries allow VPNs, etc.