@jfb Another real world use case would be for local domains that span multiple subnets. When you configure conditional forwarding, it assumes the reverse lookup subnet based on your configured router IP. In mycase i'm just using "local" but i'm using it on 3 different subnets/vlans.
I've had to create a custom file like @anon55913113 but just with additional reverse zones:
pi@ns01:~ $ cat /etc/dnsmasq.d/02-custom.conf
server=/4.81.10.in-addr.arpa/10.81.0.6