Performance. Query patterns within my network result in about 30% of all DNS responses being served from the network-local cache. This significantly reduces resolution latency for popular domains by eliminating an additional forwarding hop outside of the network.

Flexibility in blocking. A powerful rules engine for blacklisting domains has allowed very granular access control with zero client-side (device) configuration. Devices only need to be connected to my network; DHCP designates my router as the sole DNS server (which in itself uses the Pi-hole host as a single upstream). Clever clients who manually specify alternative DNS servers will have their ingress and egress traffic on port 53 routed to the Pi-hole host anyways with a router-level IP routing rule.