robot_localization and angular velocity for better odometry

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Dear all, If I input angular velocity and odometry pose data into robot_localization, can we improve odometry pose estimation due to angular velocity fusion from IMU?

Based on my understanding of robot_localization's code, it will not improve odometry pose estimation. Am I right? The reason of my assumption is that I cannot find correction of pose estimation based on angular velocity fusion in robot_localization's code. If we want to improve odometry pose estimation, we must input orientation information IMU. Am I right?

Assuming that we can improve odometry pose estimation based on orientation information, does anyone know the minimum accuracy of orientation of IMU to see the meaningful improvement? For example, if my IMU orientation has accuracy of 10 or 20 degrees, is it acceptable to achieve better odometry pose estimation?

Thanks in advance for your reading and answer.