The Kobuki base contains the IMU, motors and drive circuitry which move the Turtlebot.
The Turtlebot 1 used the iRobot Create for its motors, so you could use the Create as an alternate base, but the IMU that was used with the Tutlebot 1 is discontinued, so that will be difficult to get or find a replacement for. The Create may also be discontinued now that iRobot has introduced the Create 2. (I don't know how well the Create 2 works with ROS, and I'm not sure it will work directly with the turtlebot software).
You could also build your own base with custom motors, IMU, etc, but be prepared to spend a lot of time writing drivers for your custom base, debugging those drivers, and tuning the parameters to match whatever specific IMU you choose.
In short, if you want your robot to move, you need a mobile base. The kobuki base is a pretty good deal, and it's already integrated with the turtlebot software.