Hello Cameron
The router ospfv3 [processID] command is necessary simply to initiate the OSPF process on the router. Even if you don’t configure anything under the OSPFv3 configuration mode, it still enables the process. There are many features and mechanisms that are enabled under that configuration mode, but by default, if you just want to enable OSPFv3 adjacencies, then no additional configs are necessary.
Network statements are not used in OSPFv3. If you enable OSPFv3 on an interface, you are essentially saying that the subnet associated with that interface should participate in OSPF.
I hope this has been helpful!
Laz