In large organizations, there might be hundreds or thousands of network devices from different vendors. All of them need security updates, configuration changes, maintenance.
In vendor-driven environments, each solution of each vendor managed with specific commands or via proprietary programming interfaces. Many management systems, many people with different skills, synchronization between management processes - complexity makes it difficult and costly.
SDN streamlines dynamism in the network. Unification of hardware architectures, programming interfaces, protocols and data models makes it possible to manage different devices with unified set of management tools. It makes integration of new solutions simpler, thus faster and cheaper.
The (truly) open SDN ecosystems encourage further decomposition of functions and components. Small companies and startups are getting chance to offer their products, many are open source and free. This gives customers more choices to develop and use innovative products, build agile and cost-optimized solutions.