What do we automate with SDN? Mainly configuration changes on network devices and services. For example, some changes might be pushed as a reaction on an event like peer device down or new service instance activation via self-service portal. The difference with "pre-SDN" approaches is in implementation.
Before, management applications of some vendor mainly controlled network devices of the same vendor using proprietary interfaces and management protocols. Now, management applications control through intermediaries, which abstract implementation details using vendor-neutral interfaces, protocols and data models.
For example, consider a need to redirect data flows in reaction to link overload, changed business conditions, etc. Before, you would likely analyze network state separately, prepare network changes separately, configure devices in each domain (IP/MPLS core, metro, optical transport, data centers) separately. SDN allows to make changes automatically for end-to-end services crossing multi-vendor networks.