Hyper-V Server was released as an option for admins to run their VMs on a robust virtualization platform, with all the available features at absolutely no cost. Licensing will be covered in the next chapter, but imagine the following scenario:
You are running Linux VMs on a Hyper-V Host. If you use Windows Server (GUI, Server Core or MiniShell), you must license your OS for the host, because Windows Server will always require a license. If you use Hyper-V Server, you can run without a Windows Server License. In this case, you can have all Hyper-V features and run as many VMs as your hardware is able to run (Microsoft supports 1024 VMs per host) at absolutely no cost.
Additionally, you can use the Hyper-V Server for any supported guest OS with multiple benefits:
Minimal attack surface, as all other roles and features are not available (other features to support Hyper-V, such as Failover Clustering, are available)
Less updates, as only Hyper-V updates, and few other updates...