What is volumes: nginx_data and nginx_data:/etc/nginx/templates for in Docker? What does it give?

Question

Docker

What is the point of writing nginx_data:/etc/nginx/templates?
I read the documentation, but did not find a justification. At the university, the teacher wrote so simply (I didn’t think to clarify something then).
And now in the work task I don’t understand whether it is necessary or not ...

{"version": "3.1",
  "volumes": {
    "postgres_data": { },
    "rabbitmq_data": { },
    "nginx_data": { }
  },
...,
 "nginx": {
      "build": {
        "context": "./",
        "dockerfile": "Dockerfile-nginx"
      },
      "env_file": ".env",
      "ports": [
          "${NGINX_PORT_EXT}:${NGINX_PORT_INT}"
      ],
      "volumes": [
          "nginx_data:/etc/nginx/templates"
      ],
      "links": [
          "server",
      ]
    },