Why label k8s deployment?

Question

In the below deployment yml, the deployment itself has a label "app: hello". What would happen if I delete that line (line 6)?

I understand the functional need for pods to be labeled, because then the deployment can reference those pods by labels. In the below example, that means that I understand why lines 11-12 and 16-17 exist. But that doesn't answer why the deployment should be labeled.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: helloweb
  labels:
    app: hello
spec:
  replicas: 1
  selector:
    matchLabels:
      app: hello
      tier: web
  template:
    metadata:
      labels:
        app: hello
        tier: web
    spec:
      containers:
        - name: hello-app
          image: registry.gitlab.com/itential/cloud-team/applications/hello-app
          ports:
            - containerPort: 8080
      imagePullSecrets:
        - name: gitlab-reg

Answer 1

Labels lets you as user query for particular objects; from the docs:

Labels enable users to map their own organizational structures onto system objects in a loosely coupled fashion, without requiring clients to store these mappings.

For example if you use some kind of deployment tool like Spinnaker; the tool labels every single object that it touched so it knows about it - can manage it.

When you query for a label you can use equality-based operators (=, == and !=). K8s doc should be good starting point, alternatively check out this article on labels.