It's a single chokepoint for all DNS traffic on my network. So I do not need to configure any device or application to use it. For instance, my piece of crap Roku stick benefits from all the same ad blocking that my mac, iphone, etc... do.

There is also a lot that goes on outside of your browser. It blocks tracking logic embedded inside other things, too. For example, my TV cannot phone home to Samsung anymore to tell it what I am doing.

You can get really wild with firewall rules to truly prevent any DNS traffic from escaping your physical network.

Some not very technical reasons:

1) Run once, use everywhere. (Including devices without a browser, like an Android/Apple/'smart' TV.)

2) iPhones.

3) Configure alongside rest of networking, not clicking buttons to manually sync within browsers of the same type.

Pi hole blocks at the DNS later, so non-browsers are also affected, e.g. IoT devices.