Why would I want to use a gyroscope if I already use an accelerometer and magnetometer?

Both accelerometers and magnetometers are noisy sensors, which means they exhibit a slow response rate (due to noisy filtering) in fast moving systems. Accelerometer measurements are also affected by motion. By using a gyroscope and MotionFusion, the MotionFusion solution will have a faster response rate and track small, quick movements much more quickly. Gyroscope MotionFusion also makes it possible to sense linear acceleration separately from gravity.