The first and most minimal example of a Deployment in the Kubernetes documentation has the app: nginx line that repeats itself three times. I understand it's a tag, but I haven't found anything that explains why this needs to be specified for all of:

1. metadata.labels,
2. spec.selector.matchLabels, and
3. spec.template.metadata.labels

The example deployment file:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80