An Azure Virtual Network (VNET) is something that you will create in Microsoft Azure. The VNET will allow virtual machines and the other resources that are part of the VNET to communicate with each other privately. It is the VNET that provides this communication function. Without a VNET, or if a virtual machine resides outside the VNET, communication with other resources would not be possible. The VNET offers another level of abstraction, and thus, another layer of extra security.
VNET and virtual networking are useful and important if you want to make use of both your on-premises network, infrastructure, and resources in Microsoft's Azure cloud.
When deciding whether you need a virtual network or not, you will have to consider what your end goal is and what exactly you want to achieve. It's important to get the network set up correctly the first time, as it is much easier to deploy new virtual machines to a network than it is to add existing virtual machines...
