This will basically allow the DHCP server to only give out one IP address, and it will only give it to your HDA (if your HDA is ever configured to use DHCP, which it shouldn't). All other DHCP requests will be ignored, allowing your HDA DHCP server to be used for all computers on your local network.

BTW, it's much easier if you don't want NAT (i.e. if you have another router that does that), then you can disable both NAT and DHCP by choosing bridge mode.

You just can't disable DHCP alone.