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The transform is actually published by the robot_pose_ekf filter, which fuses the wheel odometry and IMU data to get a better result.

The joint states are used to tell where the robot's parts are. A separate node (robot_state_publisher) turns joint_states + the URDF specification into a series of transforms. On the stock turtlebot, this isn't very interesting, but if you have a turtlebot arm attached, the joint states output allows you to see the arm moving in RVIZ.