Many users actually use the Pi-hole integrated server.
The main advantage is obviously for those who cannot change the DNS server at all in their router as it is locked down by their ISPs (this includes me as well) that the Pi-hole DHCP server will correctly hand out the Pi-hole as DNS server.
Another big advantage is that all DHCP devices will immediately be known by their host names on the Pi-hole. We commonly see posts like
Since I switched to Pi-hole's DHCP server, I see host names everywhere on the dashboard where before there have only been IP addresses.
Also, you can now easily manage static leases from the dashboard but for most this may not be an advantage as they either don't use static leases (but rely on host names like
printer.local instead of some random IP address) or their routers could also do it (mine couldn't).