The most touted feature of Mac OS 9 was the new Sherlock 2 but there were lots of other new features, mostly related to the development and coming of the NeXT generation operating system, Mac OS X.

Mac OS 9 had multiple users, Voiceprint password, Keychain, automatic updating, encryption, Internet File Sharing, Internet AppleScript, and Network Browser. Many of these were direct equivalents of Mac OS X features which were concurrently developed or even backported.

The reason was of course to make the Mac OS more powerful and more modern but also to ease the transition to OS X, which at the time was believed to start in less than a year

Image taken from Toastytech.com