service selector vs deployment selector matchlabels

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