VMs talk to each other by sending information over networks. Usually the app and DB servers listen on specific ports, and in order to minimize network latencies these VMs better be communicating via the same device. This device could be a real network switch, or now more often a virtual switch running inside a hypervisor. In the traditional static network design these VMs would be using the same VLAN implemented by few tightly connected devices. However, in cloud environments configuring multiple VLANs and switches becomes a management headache.