What are docker networks are needed for? - docker

Question

Please explain why docker network is needed? I read some documentation, but on a practical example, I don't understand why we need to manipulate the network. Here I have a docker-compose in which everything works fine with and without network. Explain me please, what benefits will be in practical use if you uncomment docker-compose in the right places? Now my containers interact perfectly, there are migrations from the ORM to the database, why do I need a networks?
version: '3.4'
services:
main:
container_name: main
build:
context: .
target: development
volumes:
- .:/usr/src/app
- /usr/src/app/node_modules
ports:
- ${PORT}:${PORT}
command: npm run start:dev
env_file:
- .env
# networks:
# - webnet
depends_on:
- postgres
postgres:
container_name: postgres
image: postgres:12
# networks:
# - webnet
environment:
POSTGRES_PASSWORD: ${DB_PASSWORD}
POSTGRES_USER: ${DB_USERNAME}
POSTGRES_DB: ${DB_DATABASE_NAME}
PG_DATA: /var/lib/postgresql/data
ports:
- 5432:5432
volumes:
- pgdata:/var/lib/postgresql/data
healthcheck:
test: ["CMD-SHELL", "pg_isready -U postgres"]
interval: 1m30s
timeout: 10s
retries: 3
# networks:
# webnet:
volumes:
pgdata:

Answer 1

If no networks are defined, docker-compose will create a default network with a generated name. Otherwise you can manually specify the network and its name in the compose file.
You can read more at Networking in Compose

Answer 2

Here explained Docker network - Networking overview, and here are tutorials:
Macvlan network tutorial,
Overlay networking tutorial,
Host networking tutorial,
Bridge network tutorial.

Answer 3

Without config network for containers, all container IPs are in one range (Ex. 172.17.0.0/16). also, they can see each other by name of the container (Docker internal DNS).
Simple usage of networking in docker:
When you want to have numbers of containers in a specific network range isolated, you must use the network of docker.