Question

What are the benefits of using a redundant license server?

Answer

The main advantage is that licensing will continue to work even if the main licensing server stops working or loses network connectivity.

While using the license, the workstations will interact with the main license server, sending the license parameters and usage statistics to it.
The main license server will regularly synchronize this data with the backup server.
If the main license server fails, the backup server takes over the license management and continues to store information coming from the workstations. 
When the main server goes online, the license data is synchronized with the backup, and the main server again starts managing the licenses.
Please note, that the backup server will stop providing licenses if the main server is not restored before a certain timeout. This timeout is specified in internal license settings and can range from 4 to 5000 hours.
If the backup license server fails while the main server is functional, the system will continue to work as if there is only one license server. When the backup server is restored, it will be synchronized with the main server and will start working as a backup again.