What is the utility of GCE_VM_IP

[mountaincode2]

I am confused about zonal NEGs - particulary `GCE_VM_IP`.

I am looking at the following documents, parts of which i will cite in my query:

https://cloud.google.com/load-balancing/docs/negs/zonal-neg-concepts

In the following section, the doc says:

https://cloud.google.com/load-balancing/docs/negs/zonal-neg-concepts#gce-vm-ip

These (GCE_VM_IP) zonal NEGs contain one or more endpoints represented using the primary internal IPv4 address of a Compute Engine VM's network interface... Even though Google Cloud uses an IP address to represent the endpoint, the purpose of a GCE_VM_IP endpoint is to identify the network interface itself. 

What does this even mean espesically the part that "the purpose of a GCE_VM_IP endpoint is to identify the network interface itself":

Even though Google Cloud uses an IP address to represent the endpoint, the purpose of a GCE_VM_IP endpoint is to identify the network interface itself. The network interface must be in the NEG's subnet.

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I am unable to understand why to use a `GCE_VM_IP`. it seems that having a regular instance in an instance group should suffice.

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Can you please help me connect the dots.

Thank you!

[mountaincode2]

Update.

I found a doc that states the following:

https://cloud.google.com/load-balancing/docs/network/networklb-backend-service#backend-service-netwo...

To distribute packets to a non-nic0 interface, you must use zonal NEG backends (with GCE_VM_IP endpoints).

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But what does the following mean - why can you not specify a port if the endpoint identifies a network interface:

https://cloud.google.com/load-balancing/docs/negs/zonal-neg-concepts#gce-vm-ip

Because a GCE_VM_IP endpoint identifies a network interface, you cannot specify a port with a GCE_VM_IP endpoint.