A hybrid system combines the best of both worlds as it's an on-premises system that is hosted in the cloud. You may have a cloud based phone service or more traditional PRI lines from your provider, but all the administration, reporting and additional services reside in the cloud.
That means that all of your administrative services become available from anywhere. For example, let's say the electricity goes out in the building. You can still access those administrative services to divert calls to another office or to home phones, or mobile phones. You could even failover to a Voip solution.
With a hybrid solution you can also run your primary voice services through Voip and then failover to traditional lines. So, if you were to lose internet services you wouldn't lose a minute of availability.