service selector vs deployment selector matchlabels

Question

I understand that services use a selector to identify which pods to route traffic to by thier labels.

apiVersion: v1
kind: Service
metadata:
  name: svc
spec:
  ports:
  - name: tcp
    protocol: TCP
    port: 443
    targetPort: 443
  selector:
    app: nginx

Thats all well and good.

Now what is the difference between this selector and the one of the spec.selector from the deployment. I understand that it is used so that the deployment can match and manage its pods.

I dont understand however why i need the extra matchLabels declaration and cant just do it like in the service. Whats the use of this semantically?

apiVersion: apps/v1 
kind: Deployment
metadata:
  name: nginx
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 1 
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx

Thanks in advance

Answer 1

Simple as that - in the service spec.selector you can identify which pods to route traffic to only by their labels.

On the other hand, in the Deployment spec.selector you have two options to decide on which node the pods will be scheduled on, which are: matchExpressions, matchLabels.

let me know if it answers your question :)