What is spec - selector - matchLabels used for while creating a deployment?

Answer for this question we can find in section Deployments from kubernetes.io

So, why will I need the selectors as well?

Quotes below from documentation for k8s v 1.14

.spec.selector is an required field that specifies a label selector for the Pods targeted by this deployment.

.spec.selector must match .spec.template.metadata.labels, or it will be rejected by the API.

In API version apps/v1, .spec.selector and .metadata.labels do not default to .spec.template.metadata.labels if not set. So they must be set explicitly. Also note that .spec.selector is immutable after creation of the Deployment in apps/v1.

A Deployment may terminate Pods whose labels match the selector if their template is different from .spec.template or if the total number of such Pods exceeds .spec.replicas. It brings up new Pods with .spec.template if the number of Pods is less than the desired number.

Pods are already being started separately, but later brought under the umbrella of Deployment to be managed together?

Simply speaking, No

Note: You should not create other pods whose labels match this selector, either directly, by creating another Deployment, or by creating another controller such as a ReplicaSet or a ReplicationController. If you do so, the first Deployment thinks that it created these other pods. Kubernetes does not stop you from doing this. If you have multiple controllers that have overlapping selectors, the controllers will fight with each other and won’t behave correctly.