Network is the section in the control panel containing information about your service's IP addresses. You need this information when you want to point your custom domain to your IP address.
There are two main reasons to use private networks:
Data transfer is usually much faster between two services using a private network, because a local network is used. The ping is low and the bandwith is high.
Region-based private networks
To add two or more services to a single private network, choose the same location.