Multiple ip address on a interface advertisement

The network topology is router A and router B are directly connected with IP addresses configured on their physical interfaces int gig 0/0/1 and gig0/0/2. The third interface gig0/0/3 is configured as a layer-2 trunk port, and VLAN 12 is allowed to pass on router A and connect to router B's interface gig 0/0/3 layer-2 trunk port. The IGP ospf 100 is configured on both routers and enabled at int gig 0/0/1 and gig0/0/2.

I have multiple IP addresses on the VLAN interface as below:

interface vlanif 12
ip address
ip address sub
ip address sub
ip address sub

These are advertised in router ospf by network statements in ospf 100, but by mistake, network was skipped when configuring the OSPF process network statement. The directly connected router B does not show the route of the subinterface IP addresses of, and

What is the concept of multiple IP addresses on a interface, and where it is used?

Why does OSPF fail to advertise the subinterface IP address when the primary IP address network of VLAN 12 has been skipped in the OSPF configuration?

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