SDN can play a role in orchestrating between the physical and virtual network functions. There are use cases where an operator can maintain a physical network infrastructure for handling some minimum load. And whenever the load increases, the SDN controller can take action to reroute the traffic through virtual network function on a need basis. This also requires other components in the network for orchestrating between physical and virtual entities.
The main USP of this is the fact that operator can save on additional cost of physical network infra and can manage the quality of service with an on demand cloud based virtual infra. In case of LTE, you can think of spawning off a new virtual PGW to off load traffic from the physical PGW when the traffic load goes beyond a point. I hope this helps you to get the point.
-Shyam
