iPad Pro runs iOS 9...the simple answer (and to quote directly from Apple) is:
iOS 9 is full of enhancements you’ll appreciate every day. Your apps become more essential. New multitasking features on iPad make you even more productive. Siri can do more than ever, and new proactive suggestions help you get things done before you ask. And improvements at the foundation of the operating system enhance performance, battery life, and security. The more you do with iOS 9, the more you’ll wonder how you ever did without it.