Let`s say you have multiple datacenters, each with several subnets. One subnet for public (available on the internet) and several internal subnets.
With the new deployment features in VMM 2012, it is most likely that you will consider deploying services, which could be a combination of many VMs.One VM with SQL server installed, one VM with applications that are sequenced with Server App-V, and one VM with web applications.
Let`s also say that you want these services to be load balanced among the datacenters.
Then you`ll have to create a logical network that is associated with the subnets that is available on the internet, and connect the VMs and the service to this network.
The benefit here is that you can have multiple users/developers that manage the applications, with no need to focus on the actual network being used. They only have to associate their VMs and services with the logical network, no matter where they actually are located, since the logical network can have several subnets.
The key word here is abstraction, so that the users do not have any requirements for knowing the details about the networks actually being used.