Resource management: Group and subdivide VMs in different VNs by internal security policies (some VMs cannot reach some others VMs).
Address spaces: Different virtual networks can have different sets of addresses. A single address space can be subdivided into subnets.
Security: While internal to a single VN, each VM can reach other VMs by default. You can control inbound and outbound traffic at subnet-level VMs with Network Security Groups.
VNs in an Azure account can be found in the Ibiza portal in two distinct containers by going to Browse | Virtual Networks or Browse | Virtual Networks (Classic). From the Classic portal, you will find only Classic VNs.
You can create a VN by going to New | Networking | Virtual Network, by deciding whether you need a classic or a Resource Manager...