Amazon web services – Why is the port field in Istio Gateway important?

I’m currently learning about Istio’s Gateway, and there’s something I don’t quite understand.

So when we install istio, we get the Deployment istio-ingressgateway:

$ kubectl get deploy -n istio-system
NAME                   READY   UP-TO-DATE   AVAILABLE   AGE
istio-egressgateway    1/1     1            1           3h9m
istio-ingressgateway   1/1     1            1           3h9m
istiod                 1/1     1            1           3h9m

Now, this istio-ingressgateway is exposed via the Service:

$ kubectl get svc -n istio-system
NAME                   TYPE           CLUSTER-IP      EXTERNAL-IP                                                              PORT(S)                                                                      AGE
istio-egressgateway    ClusterIP      172.20.33.252   <none>                                                                   80/TCP,443/TCP                                                               3h10m
istio-ingressgateway   LoadBalancer   172.20.19.8     abc-xyz.us-east-1.elb.amazonaws.com   15021:30308/TCP,80:32116/TCP,443:30034/TCP,31400:30430/TCP,15443:30260/TCP   3h10m
istiod                 ClusterIP      172.20.27.14    <none>                                                                   15010/TCP,15012/TCP,443/TCP,15014/TCP                                        3h10m

Now, the proxy (istio-ingressgateway deployment’s pods) listens on port 8080, which is the targetPort for port 80 in the istio-ingressgateway Service.

Now let’s say I have the following Gateway configuration:

apiVersion: networking.istio.io/v1beta1
kind: Gateway
metadata:
  name: my-ingress
spec:
  selector:
    app: my-ingressgateway
  servers:
  - port:
      number: 80
      name: http2
      protocol: HTTP2
    hosts:
    - "www.example.com"

What I don’t understand is, why is the server’s port.number configuration is important, and on what it affects?

I’ll explain: when I provision a EKS cluster for example and install istio on it, so I get a AWS Load Balancer for that istio-ingressgateway Service. One of the ports of the load balancer is port 80, which forwards the traffic to a nodePort, which is than routed to port 8080 in the proxy container:

...
    - name: http2
      nodePort: 32116
      port: 80
      protocol: TCP
      targetPort: 8080
...

So basically, when I access <load-balancer-domain>:80, I’m routed to one of the cluster nodes to port 32116, and from there to the appropriate pod, so the pod doesn’t even know what port I as a client used to reach it.

My question is then, what does the Gateway configuration "care" about what port I use to reach the proxy container?

EDIT

I’ll try to rephrase the question:
The istio-ingressgateway pod listens on port 8080. The istio-ingressgateway Service routes traffic to this port. If so, what is the meaning of the .servers[].port configuration in the Ingress resource?

The documentation about Gateway says:

While Istio will configure the proxy to listen on these ports, it is the responsibility of the user to ensure that external traffic to these ports are allowed into the mesh.

And I ask: What do they mean by "Istio will configure the proxy to listen on these ports"? The proxy listens on the ports configured in the pod’s specification, for example 8080 for HTTP, and the Service is the one that specifies which port will forward traffic to these ports in the pod, so what is the meaning of this port number in the Ingress resource ?!?!

Hope it’s more clear now